The display of sexual behaviors by female rats administered ICI 182,780

ICI 182,780 (ICI) is a pure antiestrogen that when administered systemically does not cross the blood-brain barrier, thus its actions are limited to the periphery. Four experiments were conducted to test the effects of ICI on the display of sexual behaviors in ovariectomized rats. Experiment 1 examined the effects of three doses of ICI (250, 500, and 750 μg/rat) on sexual receptivity and paced mating behavior in rats primed with estradiol benzoate (EB) in combination with progesterone (P). Experiments 2 and 3 compared the display of sexual behaviors in rats primed with EB+P or EB alone and administered either 250 μg ICI (Experiment 2) or 500 μg ICI (Experiment 3). Experiment 4 tested the effects of ICI (250 and 500 μg) on the expression of estrogen-induced progestin receptors in the uterus. ICI did not affect the display of sexual receptivity in any experiment. In rats primed with EB+P, paced mating behavior was altered by the 500 and 750 μg, but not the 250 μg, doses of ICI. The lowest (250 μg) dose of ICI did alter paced mating behavior in rats primed with EB alone. The effects of ICI on paced mating behavior were manifested by a substantial lengthening of contact-return latencies following intromissions and ejaculations. The percentage of exits were not affected by ICI. Estrogen stimulation of uterine weight and induction of uterine progestin receptors was suppressed by ICI (250 and 500 μg). ICI effects on paced mating behavior in hormone-primed female rats are likely to reflect antiestrogenic actions in the periphery, including interference with the estrogen induction of progestin receptors.     

Different doses of estradiol benzoate induce conditioned place preference after paced mating

The ovarian hormones estrogen and progesterone are required for the complete display of sexual behavior in female rats. Paced mating produces a reward state in intact cycling and ovariectomized (OVX), hormonally primed females as evaluated by the conditioned place preference (CPP) paradigm. Most of the studies that have evaluated CPP induced by paced mating in OVX females have used relatively high doses of estradiol benzoate (EB). In the present study we determined if different doses of EB, combined with progesterone (P), could induce CPP after paced mating. For this purpose OVX female rats were divided in five groups that received one of different doses of estradiol benzoate (5, 2.5, 1.25 or 0.625 μg estradiol + 0.5 mg of progesterone) before being allowed to pace the sexual interaction and conditioned in a CPP paradigm. We found that the lowest dose of EB used (0.625 μg) significantly reduced the lordosis quotient and the lordosis coefficient. Even though these females paced the sexual interaction, they didn't change its original preference, suggesting that sexual interaction did not induce a positive affective, reward state. Females allowed to pace the sexual interaction with higher doses of EB developed CPP after paced mating. These results indicate that a threshold of estradiol is required for paced mating to induce CPP.     

Previous sexual experience alters the display of paced mating behavior in female rats

The present study tested whether the display of paced mating behavior in female rats over four weekly tests is affected by sexual experience and whether test parameters, i.e., ending the test based on time or number of stimulations received, influence behavioral changes. In Experiment 1A rats with nonpaced sexual experience returned to the male more quickly overall compared to sexually naïve rats in a 30-min test of paced mating behavior. In Experiment 1B, rats received four weekly 30-min tests with one, different, male rat partner each week. Over the four tests, rats returned to the male significantly more quickly after intromissions, but significantly more slowly after ejaculations. Experiment 2A tested whether sexual experience would influence paced mating behavior in tests with a 15-intromission end criterion and the male replaced after ejaculation. Rats tested weekly under 15-intromission test conditions returned to the male significantly more quickly after intromissions, but no behavioral change was observed after ejaculations. When those same rats were given a 30-min test of paced mating behavior (Experiment 2B), they returned to the male significantly more slowly after ejaculations. Collectively, these data show that sexual experience influences the display of paced mating behavior in female rats and that the test parameters interact with sexual experience to influence the nature of the changes. Sexual experience may facilitate behaviors that promote reproductive success in female rats.     

Systemic ICI 182,780 alters the display of sexual behaviors in the female rat

The present study investigates the effects of the antiestrogen ICI 182,780 (ICI) on the display of sexual behaviors in female rats. ICI 182,780 is a pure anti-estrogen and when given systemically, ICI is thought to act only in the periphery, and is not believed to cross the blood brain barrier. The present study examines the effects of ICI on sexual receptivity and on paced mating behavior following treatment with estradiol benzoate (EB) and progesterone (P) (Experiment 1) or with EB alone (Experiment 2). In Experiment 1, ICI (250.0 microg) did not affect the display of receptivity or paced mating behavior induced by EB and P. In contrast, in Experiment 2 female rats receiving EB alone displayed a decrease in the level of sexual receptivity following treatment with 500.0 and 750.0 microg ICI (but not 250.0 microg ICI). In addition, in Experiment 2 EB-treated female rats receiving 250.0 microg ICI spent more time away from the male rat following an intromission and were more likely to exit from the male compartment following a mount. Last, ICI had potent antiestrogenic effects on vaginal cytology (Experiment 2) and on the uterus (Experiments 1 and 2). The present study supports a role for peripheral estrogen receptors in sexual receptivity and paced mating behavior and suggests that estrogen receptor activation may decrease the aversive sensation associated with sexual stimulation.     

A soy supplement and tamoxifen inhibit sexual behavior in female rats

In addition to displaying proceptive (hopping and darting) and receptive (lordosis) behaviors during a sexual encounter with a male, female rodents will regulate the timing of the encounter by engaging in a series of approaches and withdrawals from the male, a behavior termed paced mating behavior. Proceptive, receptive, and paced mating behaviors are all regulated by, and sensitive to, estrogen and progesterone, suggesting that compounds capable of disrupting these critical hormones may also perturb the display of female sexual behavior. The present experiments examined the impact of the selective estrogen receptor modulator (SERM) tamoxifen and a popular soy phytoestrogen dietary supplement on female sexual behavior in rats. Ovariectomized female rats were given either tamoxifen (TAMOX) by implant or the soy supplement through the diet then injected with estradiol benzoate (EB, 10 microg) or oil followed 48 h later with an injection of progesterone (P, 500 microg). Animals were then tested for sexual behavior 4 h after the P injection. Neither compound had any effect on sexual behavior when administered in conjunction with P alone; however, both significantly diminished receptive behavior, as measured by the lordosis quotient (LQ), in animals primed with both EB and P. Similarly, the hopping and darting rate was also significantly depressed in both the soy- and TAMOX-treated animals, compared to the EB- and P-treated controls, with the soy-treated animals showing significantly less proceptive behavior than the TAMOX-treated animals. Finally, soy but not TAMOX significantly attenuated paced mating behavior in animals compared to the EB- and P-treated controls. These results demonstrate that both the soy supplement and TAMOX act as estrogen antagonists on both proceptive and receptive behavior in female rats.     

Paced mating behavior in female rats in response to different hormone priming regimens

A female rat will display a repertoire of behaviors during a sexual encounter with a male rat including sexually receptive (the lordosis response) and proceptive (hopping, darting) behaviors. In addition, when given the opportunity, a sexually receptive female rat will approach and withdraw from the male rat, controlling the timing of the receipt of mounts, intromissions, and ejaculations, a behavior known as paced mating behavior. The present experiments tested the hypotheses (1) that progesterone regulates paced mating behavior, and (2) that multiple hormone regimens used previously to induce sexual receptivity have the same effect on paced mating behavior. Paced mating behavior was assessed in sexually receptive ovariectomized female rats after treatment with: (1) estradiol benzoate (EB; 30.0 mg/kg) followed by a range of doses of progesterone (P; 1.0-8.0 mg/kg), (2) two pulses of unesterified estradiol (E2; 2.0 microg/rat) followed by 1.0 mg/rat of P, and (3) EB alone (5.0 microg/rat) for 6 days. No differences in sexual receptivity or in paced mating behavior were observed across doses of P (1.0-8.0 mg/kg). In contrast, the number of hops and darts per min increased with the dose of P administered. E2 + P administration resulted in slightly, but significantly, lower levels of sexual receptivity along with significantly longer contact-return latencies following an intromission in relation to the other treatment conditions. In addition, female rats exhibited fewer hops and darts per min in response to E2 + P than in response to EB + 8.0 mg/kg of P. The administration of EB alone for 6 days induced levels of receptivity and paced mating behavior indistinguishable from EB + P, while eliciting significantly fewer hops and darts per min than the EB + 8.0 mg/kg P treatment condition. Hormone priming regimen had no effect on the percentage of exits displayed during the paced mating tests in any experimental phase. Dose of P had no effect on paced mating behavior in sexually receptive rats. In addition, P does not appear to be necessary for the display of paced mating behavior following long-term treatment with EB. In contrast, the pulsatile administration of E2 + P induced a different pattern of paced mating behavior in sexually receptive rats.     

Yes, I am ready now: differential effects of paced versus unpaced mating on anxiety and central oxytocin release in female rats

Sexual activity and partner intimacy results in several positive consequences in the context of stress-coping, both in males and females, such as reduced state anxiety in male rats after successful mating. However, in female rats, mating is a rewarding experience only when the estrous female is able to control sexual interactions, i.e., under paced-mating conditions. Here, we demonstrate that sex-steroid priming required for female mating is anxiolytic; subsequent sexual activity under paced mating conditions did not disrupt this anxiolytic priming effect, whereas mating under unpaced conditions increased anxiety-related behavior. In primed females, the release of the neuropeptide oxytocin (OT) within the hypothalamic paraventricular nucleus was found to be elevated and to further increase during paced, but not unpaced mating. Central administration of an OT receptor antagonist partly prevented priming/mating-induced anxiolysis indicating the involvement of brain OT in the anxiolysis triggered by priming and/or sexual activity.These findings reveal that the positive consequences of mating in females are dependent on her ability to control sexual interactions, and that brain OT release is at least in part the underlying neurobiological correlate.     

Hormonal and testing conditions for the induction of conditioned place preference by paced mating

The ability to control or pace the sexual interaction has important physiological and behavioral consequences for the female rat. Paced mating favors reproduction and induces a positive affective state as revealed by conditioned place preference (CPP). In the present experiment we evaluated: 1) If paced mating induces CPP in naturally cycling females; 2) If females developed a positive affective state if they paced the sexual interaction through a 1- or a 3-hole pacing chamber; 3) If females that mate with the same male without pacing the sexual interaction develop CPP. In the first experiment intact females were divided in 4 different groups; 2 paced the sexual interaction until receiving 1 or 3 ejaculations; the other 2 groups mated, without pacing the sexual interaction, until receiving 1 or 3 ejaculations. Only the group that paced the sexual interaction until receiving 3 ejaculations developed a positive affective state. In experiments 2 and 3 hormonally treated ovariectomized females were used. In experiment 2 females were allowed to pace the sexual interaction through a 1- or a 3-hole pacing chamber: A clear positive affective state was induced in both testing conditions. Finally, in experiment 3 females did not develop CPP for non-paced sex despite the fact that they mated with the same male in the conditioning sessions. These results demonstrate that the pattern of vaginocervical stimulation that the females received by engaging in approach and avoidance behaviors to pace the sexual interaction can induce a positive affective state in naturally cycling females. They also confirm the existence of a threshold of vaginocervical stimulation for paced mating to induce CPP in female rats.     

Prenatal stress alters reproductive responses of rats in behavioral estrus and paced mating of hormone-primed rats

Adult female offspring of dams exposed to gestational stress (prenatal stress, PNS) may show altered reproductive behavior, exploration in novel environments, and/or social interactions than do their non-PNS counterparts. These behavioral differences may be more readily observed in a seminatural, paced mating paradigm, in which females have greater control of their sexual contacts, than in a standard mating situation. Adult offspring of dams exposed to restraint and lights for 45 min on Gestational Days 14-20 (PNS) were compared with those not subjected to stress (non-PNS, control condition). The motor, reproductive, and sociosexual behaviors of hormone-primed (Experiment 1) or cycling adult offspring in behavioral estrus (Experiment 2) were examined following 20 min of restraint stress under bright lights (postnatal stress). Hormone-primed PNS rats displayed less motor behavior in a novel arena than did non-PNS rats. In a standard mating test, hormone-primed PNS females tended to be more aggressive toward the male than were non-PNS rats. In a seminatural mating situation, hormone-primed PNS females showed increased avoidance behavior, such as longer latencies to the initial intromission, greater return latencies following mounts and intromissions, and more exiting subsequent to mounts and intromissions, than did non-PNS rats. PNS rats in behavioral estrus had decreased incidence and intensity of lordosis, and fewer solicitation behaviors, in both standard or paced mating situations, in which latency to and number of mounts were also increased. Thus, hormone-primed PNS rats exposed to restraint showed more avoidance behaviors in paced mating situations, while cycling PNS rats in behavioral estrus had greater disruption of reproductive responses in standard or paced mating paradigms than did non-PNS control rats.     

Alterations in plasma concentrations of testosterone, LH, and prolactin associated with mating in the male rat.

The hormonal response of the male rat to sexual activity was investigated in two studies. In the first, no evidence of a chronic elevation in plasma levels of testosterone (T), LH, or prolactin (PRL) was observed in sexually experienced rats compared to naive controls. Both groups showed an acute increase in plasma levels of all three hormones following mating, but the increases shown by the experienced group were more pronounced. In the second study, plasma levels of T, LH and PRL rose in sexually experienced male rats following exposure to a mating arena whether it contained an estrous female, an anestrous female, or no other animal. However, the increases were considerably larger in the group exposed to estrous females. It is suggested that plasma hormones rise in anticipation of mating, although not to the same extent as following mating, and that the anticipatory rise may function to initiate or facilitate mating behavior.     

Female rats exhibit a conditioned place preference for nonpaced mating

Paced, but not nonpaced, mating behavior is reported to induce a conditioned place preference (CPP) in female rats. Contrary to these previous findings, Experiment 1 showed that female rats that received 15 intromissions from a single male rat during each of five conditioning sessions exhibited a CPP for the compartment associated with mating when the intromissions were delivered via a paced or nonpaced paradigm. Experiment 2 demonstrated that nonpaced mating induced a CPP when a single male delivered the 15 intromissions but not when the male was replaced following ejaculation and a new male allowed to complete the requisite number of intromissions. These findings invite reevaluation of the reinforcing aspects of mating behavior in female rats.     

Maternal influences on the sexual behavior and reproductive success of the female rat

In many species, including humans, there is evidence for parental effects on within-sex variations in reproductive behavior. In the present studies we found that variations in postnatal maternal care were associated with individual differences in female sexual behavior in the rat. Females born to and reared by dams that showed enhanced pup licking/grooming (i.e., High LG mothers) over the first week postpartum showed significantly reduced sexual receptivity and alterations in the pacing of male mounting (i.e., longer inter-intromission intervals) observed in a paced mating test. There were minimal effects on the sexual behavior of the male offspring. The female offspring of High LG mothers showed a reduced lordosis rating, a decreased mount:intromission ratio, received fewer ejaculations and were less likely to achieve pregnancy following mating in the paced mating context. The data suggest maternal influences on the sexual development of the female rat that are functionally relevant for reproductive success. Together with previous studies these findings imply that maternal care can ‘program’ reproductive strategies in the female rat.     

Methamphetamine-enhanced female sexual motivation is dependent on dopamine and progesterone signaling in the medial amygdala

Methamphetamine (METH) is a psychomotor stimulant strongly associated with increases in sexual drive and impulsive sexual behaviors that often lead to unsafe sexual practices. In women METH users, such practices have been associated with increases in unplanned pregnancies and sexually transmitted diseases. Despite this significant heath concern, the neural mechanisms underlying this drug–sex association are not known. We previously established a rodent model of METH-facilitated female sexual behavior in which estradiol and progesterone interact with METH to increase motivational components of female behavior and neuronal activation in the posterodorsal medial amygdala (MePD) (Holder et al., 2010; Holder and Mong, 2010). The current study more directly examines the mechanisms underlying the drug–sex interaction. Here, we hypothesize that METH-induced increases in MePD dopamine signaling bridge the METH–hormone interaction. In support of this hypothesis, we found that excitotoxic lesions targeted to the MePD attenuated the METH-induced increases in proceptive behavior. Furthermore, infusion of a D1 agonist into the MePD increased proceptive behavior, while infusion of a D1 antagonist blocked the ability of METH to increase proceptive behaviors. Additionally, we found that METH-treatment increased progesterone receptor (PR) immunoreactivity in the MePD, suggesting an interaction between dopamine and progesterone signaling. Indeed, infusions of the PR antagonist, RU486, prevented METH-induced increases in sexual behavior. Thus, taken together, the current findings suggest that dopamine in the MePD modulates enhanced sexual motivation via an amplification of progesterone signaling and contributes to a better understanding of the neurobiology of drug-enhanced sexual behaviors.     

The effects of prenatal PCBs on adult female paced mating reproductive behaviors in rats

Polychlorinated biphenyls (PCBs) are a family of toxicants that persist in measurable quantities in human and wildlife tissues, despite their ban in production in 1977. Some PCB mixtures can act as endocrine disrupting chemicals (EDCs) by mimicking or antagonizing the actions of hormones in the brain and periphery. When exposure to hormonally active substances such as PCBs occurs during vulnerable developmental periods, particularly prenatally or in early postnatal life, they can disrupt sex-specific patterning of the brain, inducing permanent changes that can later be manifested as improper sexual behaviors. Here, we investigated the effects of prenatal exposure to the PCB mixture Aroclor (A) 1221 on adult female reproductive behaviors in a dose-response model in the Sprague-Dawley rat. Using a paced mating paradigm that permits the female to set the timing of mating and control contact with the male during copulation, we were able to uncover significant differences in female-typical sexual activities in A1221-exposed females. Specifically, A1221 causes significant effects on mating trial pacing, vocalizations, ambulation and the female's likelihood to mate. The results further demonstrate that the intermediate treatment group has the greatest number of disrupted endpoints, suggestive of non-linear dose responses to A1221. These data demonstrate that the behavioral phenotype in adulthood is disrupted by low, ecologically relevant exposures to PCBs, and the results have implications for reproductive success and health in wildlife and women.     

Sensory cues mediating mating-induced potentiation of sexual receptivity in female rats

Repeated mating of estradiol-primed female rats increases sexual receptivity. Two studies were conducted to determine the contribution of vaginal--cervical stimulation (VCS) to this increase. In the first study, female rats were repeatedly mated for 165 min. The vaginas of half of the females were covered with tape (masked) to prevent intromissions by the males. The remaining females were unmasked. Only females receiving intromissions (unmasked) showed a significant increase in sexual receptivity during repeated mating, suggesting that VCS from intromissions is necessary for repeated mating to increase sexual receptivity. In the second experiment, female rats received either experimentally administered VCS or control scapular stimulation administered with a plastic probe 1 h prior to testing for sexual receptivity. VCS applied in this manner significantly increased sexual receptivity. Together, these findings suggest that VCS from intromissions is one of the primary factors responsible for increases in sexual receptivity following repeated mating.     

Only Self-Paced Mating Is Rewarding in Rats of Both Sexes

When rats are mated in a traditional mating chamber (with one male and one female) in which the male dictates the pace of the copulatory sequence, males develop a reward state as evaluated by conditioned place preference (CPP). In this mating situation no reward state is induced in females. However, when female rats are able to control (pace) the rate of sexual stimulation, thereby reducing the aversive consequences associated with mating, a clear CPP is observed. In the present study the CPP paradigm was used to determine whether if the reinforced state induced by coital interactions in male rats can be maintained when females pace the sexual interaction. Adult male and female rats were mated in one of two different conditions: (1) where subjects were able to pace their coital interactions or (2) where subjects were not able to pace their sexual contacts. The results showed that when males had control over the sexual interaction they developed a clear place preference while males that mated with females that paced their coital contacts did not develop CPP. Similarly, only females that were able to pace their sexual contacts developed place preference. These results suggest that coital interactions in males, as well as in females, can induce a reward state only when they are able to control the sexual interaction. Under seminatural conditions sexual behavior in rats is highly promiscuous, they mate in groups and repeatedly change partners in the middle of copulation. This behavioral sequence allows both, male and female to control the rate of sexual interaction, assuring the induction of a reward state outlasting the actual performance of coital responses.     

Effects of subcutaneous injection and intrahypothalamic infusion of releasing hormones upon lordotic response to repetitive coital stimulation.

The effects of luteinizing hormone-releasing hormone (LRH) and thyrotropin-releasing hormone (TRH) upon the lordotic response to repetitive coital stimlation were studied using ovariectomized (OVX) and ovariectomized-adrenalectomized (OVX-ADX) female rats. Both OVX and OVX-ADX rats, pretreated with estrone alone, exhibited a dual behavioral response to repeated coital stimulation. The initial response to short-term stimulation was facilitatory with peak sexual receptivity occurring approximately 120 min following the initial male contact. This initial phase was followed by a depression of sexual receptivity associated with continued coital stimulation. Subcutaneous injection of 500 ng of LRH prior to mating was found to significantly potentiate the initial increases in sexual receptivity and to delay the onset of behavioral depression. The injection of 500 ng of TRH was observed to significantly depress behavioral enhancement due to repetitive coital stimulation.The repetitive coital stimulation model was utilized to localize forebrain areas behaviorally responsive to LRH and TRH. Stainless steel cannulas were implanted into either the medial preoptic area (MPOA), arcuate area (ARC), lateral hypothalamic area (LHA), or cerebral cortex (CC). Cannulated animals, primed with estrone, were tested for sexual receptivity immediately prior to experimental treatment, i.e., the infusion of 0.5 μl of 50 ng of LRH or TRH in 0.9% saline, 0.5 μl of 0.9% saline, or sham infusion. A second mating (postinfusion) test was performed 1.75 hr following infusion. When infused into the MPOA or ARC, LRH significantly enhanced lordotic behavior as compared to values obtained for saline or sham infusions. The infusion of LRH into LHA or CC showed no enhancement beyond the levels observed in control infusions (saline and sham infusions). The infusion of TRH into the MPOA or ARC depressed lordotic enhancement to repeated mating, however, this depression was significant only in ARC. These findings were consistent with previously demonstrated actions of releasing hormones upon neural activity within the MPOA and ARC.     

Self-control and honesty depend on exposure to pictures of the opposite sex in men but not women

Research has shown that viewing stimuli that induce mating or sex motivation can push men towards greater impulsivity, a manifestation of lower self-control. Recent advances in research on the connection between self-control and moral behavior indicate that low self-control is associated with increased dishonesty. From an evolutionary perspective, when mating motivation is activated, men may behave in dishonest ways by projecting characteristics in line with women's mate preferences to enhance their sexual attractiveness. We tested the possibility that exposure to pictures of sexually appealing women would engender lower self-control, leading men to behave dishonestly. The results showed that a state of lower self-control was observed in males who viewed women rated high on sexual attractiveness but not in males who viewed women rated low on sexual attractiveness or in females who viewed men (Experiment 1). Compared with control participants, male participants exposed to pictures of sexy women were less likely to return excess money received for participating (Experiment 2) and more likely to cheat in a matrix task (Experiments 3 and 4). State self-control mediated the link between exposure to sexual stimuli and dishonest behavior in men (Experiments 2 and 4). For men whose mating motivation is heightened by exposure to sexual stimuli, dishonesty appears to be a tactic for projecting characteristics preferred by women (e.g., large economic resources).     

Sex and context: hormones and primate sexual motivation

Gonadal hormones regulate the ability to copulate in most mammalian species, but not in primates because copulatory ability has been emancipated from hormonal control. Instead, gonadal hormones primarily influence sexual motivation. This separation of mating ability from hormonally modulated mating interest allows social experience and context to powerfully influence the expression of sexual behavior in nonhuman primates, both developmentally and in adulthood. For example, male rhesus monkeys mount males and females equally as juveniles, but mount females almost exclusively as adults. Having ejaculated with a female better predicted this transition to female mounting partners than did increased pubertal testosterone (T). It is proposed that increased pubertal T stimulates male sexual motivation, increasing the male's probability of sexual experience with females, ultimately producing a sexual preference for females. Eliminating T in adulthood reduces male sexual motivation in both humans and rhesus monkeys, but does not eliminate the capacity to engage in sex. In male rhesus monkeys the effects of reduced androgens on sexual behavior vary with social status and sexual experience. Human sexual behavior also varies with hormonal state, social context, and cultural conventions. Ovarian hormones influence female sexual desire, but the specific sexual behaviors engaged in are affected by perceived pregnancy risk, suggesting that cognition plays an important role in human sexual behavior. How the physical capacity to mate became emancipated from hormonal regulation in primates is not understood. This emancipation, however, increases the importance of motivational systems and results in primate sexual behavior being strongly influenced by social context.     

Hormone release during computer-monitored sexual behavior in mature and aged male rats

Sexual behavior and the increase in plasma hormone levels of LH, prolactin, and testosterone associated with sexual behavior were examined in three age groups of sexually naive male rats. The two younger groups (5- and 11-month-old) mated normally and their behavioral latencies decreased significantly following sexual experience. Both plasma testosterone and LH concentrations increased significantly following entrance of a receptive female into the mating arena. Plasma prolactin levels rose but not significantly. However, the 27-month-old rats neither mated nor showed an increase in the three plasma hormone concentrations during exposure to a receptive female. Only basal testosterone levels were significantly lower than those of the younger animals. Low testosterone levels possibly contributed to deficiencies in both behavior and its associated hormone release. The monitoring of sexual behavior was facilitated by a computer, programmed to record, store, and analyze behavioral events.