Mounting and receptive behavior in the ovariectomized female rat: Influence of estradiol,dihydrotestosterone, and genital anesthetization

Two experiments were performed with ovariectomized female rats in an attempt to determine whether estradiol and dihydrotestosterone work synergistically in the brain to activate mounting behavior. In Expt 1, performed in Göteborg, it was found that females treated daily with 2 μg estradiol benzoate (EB) combined with 500 μg dihydrotestosterone (DHT) displayed significantly more mounts with pelvic thrusting than other females treated with the oil vehicle, 500 μg DHT, or 2 μg EB. The behavior of rats receiving EB + DHT was indistinguishable from that of yet another group of females which received 200 μg testosterone propionate (TP). In Expt 2, performed in Rotterdam, it was found that ovariectomized female rats treated with either 200 μg TP or 2 μg EB + 200 μg dihydrotestosterone propionate (DHTP) mounted significantly more than females treated with 2 μg EB. Both clitoral size and the growth of cornified papillae on the glans clitoris were stimulated by the administration of TP or EB + DHTP. However, in no group was the frequency of mounting affected by anesthetization of the clitoris and external vagina with lidocaine paste. Lordosis quotients of females treated with EB + DHTP were significantly lower than in rats receiving either EB or TP, again regardless of whether or not the genital region was anesthetized. It is concluded that the effects of DHT on estradiol-induced mounting and receptivity most likely result from the action of this androgen on the brain, and not from the stimulatory effect which DHT may exert on genital sensory receptors.     

Effects of testosterone, estrogen, and dihydrotestosterone upon aggressive and sexual behavior of female rats

Groups of female TMD rats were treated either with estradiol benzoate (EB), dihydrotestosterone propionate (DHTP), testosterone propionate (TP), EB + DHTP (EB/DHTP), or with oil. These groups of females were tested for social aggression and for masculine and feminine sexual behavior. In addition, patterns of masculine and feminine sexual responses during the aggressive encounters, were investigated. TP-treated females of the same strain were used as opponents in the tests for aggression. In accordance with previous results, EB did not activate aggression whereas TP treatment resulted in a significant increase in aggression in females. Aggressive responses were activated by adding DHTP to EB, up to levels equal to those activated by TP. Sexual responses were observed in the tests for aggression as well as in tests for sexual behavior. The results indicated that feminine and masculine sexual responses were affected significantly by hormonal treatment. Mounting behavior in the test for aggression was activated by TP and by EB/DHTP. Lordosis and proceptive responses were inhibited in these groups as compared to EB-treated females, both in tests for aggression and in tests for sexual behavior. The results are consistent with the idea that dihydrotestosterone inhibits feminine and activates masculine sexual activity. The results also indicate that EB and DHTP synergistically activate aggression.     

Sexual differentiation of behavior in the zebra finch: effect of early gonadectomy or androgen treatment

Treatment of nestling zebra finches with estradiol benzoate (EB) has been shown to masculinize singing in females and demasculinize copulatory behavior in males, suggesting that sexual differentiation of these behaviors is under hormonal control such that testicular hormones induce the capacity for song and ovarian hormones suppress the capacity for mounting. Two experiments were carried out to obtain a more complete picture of sexual differentiation in this species. In Experiment 1, nestlings were injected daily for the first 2 weeks after hatching with testosterone propionate (TP), dihydrotestosterone propionate (DHTP), or a combination of DHTP and EB. As adults, birds were gonadectomized and implanted with TP prior to testing, then tested again after implantation with EB. Singing was not increased in females by any of the treatments. The only effect of either TP or DHTP given alone was defeminization of female proceptive behavior by DHTP. Thus androgens appear to have less influence than estrogens on sexual differentiation of behavior in this species. The combination of DHTP and EB demasculinized mounting in males. In Experiment 2, nestlings were gonadectomized at 7-9 days of age and implanted with TP prior to testing in adulthood. Early gonadectomy had little effect on later behavior; early castrated males sang, danced, and copulated normally and early ovariectomized females neither sang nor mounted.     

Estrogen plus androgen induced mounting in adult female hamsters

This study demonstrated that the combined administration of estrogens and androgens activates the display of mounting by female hamsters. Forty-nine ovariectomized hamsters were injected daily with either estradiol benzoate (EB, N = 8); dihydrotestosterone propionate (DHTP, N = 7); testosterone propionate (TP, N = 6); androstenedione (AD, N = 9); EB plus DHTP (N = 10); or estrone plus DHTP (E₁ + DHTP, N = 9). All androgens were administered at a dose of 1 mg per day for the first 24 days and at a dose of 2 mg per day for the last 14 days. The EB dose was 6 μg per day and the E₁ dose was 100 μg per day. Females were tested for male behavior once a week starting on Day 10 of injections and for female behavior on Day 39.One hundred percent of EB + DHTP treated females; 67% of the E₁ + DHTP treated females; 55% of the AD treated females; 33% of the TP treated females; 29% of the DHTP treated females; and none of the EB treated females mounted during at least one test. Only one of the E₁ + DHTP treated females showed the intromission pattern; otherwise most females which mounted displayed the intromission pattern. The median number of days preceding the onset of mounting ranged from 21 to 31 days and did not differ among treatment groups.     

Mounting behavior of female guinea pigs after prenatal and adult administration of the propionates of testosterone, dihydrotestosterone, and androstanediol.

Female guinea pigs were exposed prenatally (Day 28–58) to the propionates of testosterone (TP), dihydrotestosterone (DHTP), or androstanediol (ADP). Only TP females failed to display lordosis in adulthood after estrogen and progesterone treatment. When given TP in adulthood, females in all groups mounted, but TP and DHTP females showed augmented intromission frequencies and higher percentages of correctly oriented mounts relative to controls. Moreover, TP females responded more quickly to TP injections in adulthood and had higher over-all mounting frequencies than other groups, while DHTP females displayed mounting frequencies intermediate to controls and TP females. ADP females were not different from controls for any measure of mounting behavior.No female in any group mounted when given DHTP in adulthood, even after 7 wk of daily injections. Since male guinea pigs do mount in response to DHTP given in adulthood, the results raise the possibility that mechanisms determining sensitivity to specific steroids may not be mediated exclusively by steroids during critical periods of embryological differentiation.     

Effects of estradiol benzoate, estrone, and propionates of testosterone or dihydrotestosterone on sexual and related behaviors of ovariectomized rhesus monkeys

Ovariectomized adult rhesus monkeys were injected daily for 10 days with either 1 mg of dihydrotestosterone propionate (DHTP), 1 mg of testosterone propionate (TP), 10 μg of estradiol benzoate (EB), or 500 μg of estrone (El). On the 5th and 10th days of treatment, females received two 24-min behavioral tests with each of two adult males. All females received every hormonal treatment during the course of the study, with the order of treatments counterbalanced. Prior to the initiation of an hormonal treatment, each subject received two tests with no hormone treatment (NORX). Three behaviors related to female proceptivity were recorded. Treatment with DHTP had no influence on any aspect of proceptivity measured, in comparison to the NORX condition, whereas El or TP treatment augmented the frequencies of two of the proceptive behaviors and EB increased all three. The response of the male toward the female was influenced by the female's hormonal condition. Treatment with TP or DHTP did not increase the frequency of male contact or the mount rate in comparison to the NORX condition, whereas EB or El treatment did. In addition, DHTP was the only steroid which failed to increase the percentage of tests with intromission or ejaculation when compared to NORX. Female receptivity, as measured by acceptance or rejection of male contacts, was not different for the NORX-, TP-, EB-, or El-treated conditions. DHTP treatment, however, reduced female receptivity in comparison to all other conditions. Treatment with DHTP or TP resulted in an increase in the frequency of female yawning behavior, whereas neither estrogen treatment showed any effect on this behavior. The influences of TP on female proceptive and male sexual behavior were never duplicated or even approximated by treatment of females with the nonaromatizable DHTP. Nor was there any evidence that TP inhibited female receptivity below the level characteristic of NORX females, as was true for DHTP.     

Neonatal dihydrotestosterone and estrogen stimulation: Effects on sexual behavior of male rats

Male rats castrated on the second day after birth (Day 2) were, for the next 10 days, given daily injections of one of five steroids or steroid combinations: 200 μg of testosterone propionate (TP); 200 μg of dihydrotestosterone propionate (DHTP); 5 μg of estradiol benzoate (EB); 5 μg of estradiol benzoate plus 200 μg of dihydrotestosterone propionate; oil vehicle (VH). Control male rats castrated on Day 90 received a sham castration and oil vehicle in the neonatal period. All animals were given TP in adulthood and tested for male sexual behavior. There was no difference in mounting activity among the subjects. Day 2 DHTP subjects displayed intromissions but were incapable of ejaculating. The more frequent display of intromissions by Day 2 DHTP animals in comparison to Day 2 VH animals could be solely due to their larger and more highly developed penes. On the other hand, the ejaculatory failure of the Day 2 DHTP subjects was attributed to some deficiency in central neural processes controlling ejaculatory mechanisms rather than inadequate penile development. Equivocal results were obtained with the Day 2 EB and Day 2 EB-DHTP animals in that only a few animals in both groups showed an ejaculatory pattern.     

Effects of exogenous steroids on androgen receptors in fetal guinea pig brain

We treated pregnant guinea pigs on Day 50 of gestation with 10 mg testosterone propionate (TP), obtaining fetuses 2, 4, 8, or 18 h later as well as after 5 days of treatment. In a second group of pregnant guinea pigs, dihydrotestosterone propionate (DHTP), estradiol benzoate (E2B), progesterone (P), or cortisol was given 2 h before obtaining fetuses. Although TP treatment elevated fetal serum T (p less than 0.05), brain cytosolic androgen receptor (ARc) content was unchanged in fetuses of either sex. In female fetuses, nuclear androgen receptors (ARn) increased 10-fold in medial-basal hypothalamus (MBH) and preoptic area (POA) at 2 and 4 h (respectively) after treatment, while fetal male ARn content was unchanged. Maternal injection of other steroids (E2B, P, or cortisol, but not DHTP) significantly increased these hormones in the fetus 2 h later (p less than 0.05). Only androgens affected fetal androgen receptor (AR) content. While TP increased ARn in female MBH, DHTP decreased ARc in fetal anterior pituitary of both sexes. In this latter case, a metabolite of DHT may mediate the effects. We conclude that T crosses the guinea pig placenta and activates ARn in POA and MBH of female fetuses; male ARn appear to be maximally occupied by endogenous T. Steroids of other classes do not induce AR responses in fetal guinea pig brain. These AR changes may represent an initial cellular mechanism in brain sexual differentiation.     

Organizational effects of testosterone,estradiol, and dihydrotestosterone on vasopressin mRNA expression in the bed nucleus of the stria terminalis

In adulthood, male rats express higher levels of arginine vasopressin (AVP) mRNA in the bed nucleus of the stria terminalis (BST) than do female rats. We tested whether this sex difference is primarily due to differences in neonatal levels of testosterone. Male and female rats were gonadectomized on the day of birth and treated with testosterone propionate (TP) or vehicle on postnatal days 1, 3, and 5 (P1, P3, and P5). Three months later, all rats were implanted with testosterone‐filled capsules. Two weeks later, brains were processed for in situ hybridization to detect AVP mRNA. We found that neonatal TP treatment significantly increased the number of vasopressinergic cells in the BST over control injections. We then sought to determine the effects of testosterone metabolites, estradiol and dihydrotestosterone, given alone or in combination, on AVP expression in the BST. Rat pups were treated as described above, except that instead of testosterone, estradiol benzoate (EB), dihydrotestosterone propionate (DHTP), a combination of EB and DHTP (EB+DHTP), or vehicle was injected neonatally. Neonatal treatment with either EB or EB+DHTP increased the number of vasopressinergic cells in the BST over that of DHTP or oil treatment. However, treatment with DHTP also significantly increased the number of vasopressinergic cells over that of oil treatment. Hence, in addition to bolstering evidence that estradiol is the more potent metabolite of testosterone in causing sexual differentiation of the brain, these data provide the first example of a masculinizing effect of a nonaromatizable androgen on a sexually dimorphic neuropeptide system. © 2003 Wiley Periodicals, Inc. J Neurobiol 54: 502–510, 2003     

The influence of androgenic and estrogenic hormones on sexual behavior in castrated adult male pigs

The objectives of these studies were to evaluate the influence of testosterone propionate (TP), estradiol cypionate (EC), dihydrotestosterone propionate (DHTP), EC + TP, EC + DHTP, and TP + DHTP on traits of masculine sexual behavior in castrated adult male pigs of different breeds. Masculine sexual behavior was restored and maintained by TP, whereas EC initially activated sexual behavior, including copulation and ejaculation, but was unable to sustain copulatory behavior for the 8- to 18-week periods that were evaluated. Treatment with DHTP was ineffective for stimulation of sexual behavior; thus, it is suggested that testosterone promotes some aspects of masculine sexual behavior in male pigs via aromatization to estrogen, but both androgen and estrogen are required for maintenance of the full complement of masculine sexual behavior traits.     

Male sexual behavior in deer mice (Peromyscus maniculatus) following castration and hormone replacement

Sexually experienced male deer mice (Peromyscus maniculatus bairdi) were castrated and tested for male sexual behavior. In the weeks following castration male sexual behavior decreased. Ejaculation disappeared first, followed by intromission and, finally, mounting. Castrated males failing to copulate were assigned to one of four treatment groups: 200 μg testosterone propionate (TP); 200 μg dihydrotestosterone propionate (DHTP); 2 μg estradiol benzoate (EB); or sesame oil (OIL). TP and DHTP were equally effective in restoring the complete male sexual behavior pattern. In contrast, EB was effective in stimulating mounting and minimally effective in stimulating intromissions (vaginal penetration), but did not stimulate ejaculatory responses. These data indicate that in deer mice testosterone may mediate male sexual behavior through reduction to dihydrotestosterone rather than through aromatization to estradiol.     

Organizational effects of testosterone,estradiol, and dihydrotestosterone on vasopressin mRNA expression in the bed nucleus of the stria terminalis

In adulthood, male rats express higher levels of arginine vasopressin (AVP) mRNA in the bed nucleus of the stria terminalis (BST) than do female rats. We tested whether this sex difference is primarily due to differences in neonatal levels of testosterone. Male and female rats were gonadectomized on the day of birth and treated with testosterone propionate (TP) or vehicle on postnatal days 1, 3, and 5 (P1, P3, and P5). Three months later, all rats were implanted with testosterone-filled capsules. Two weeks later, brains were processed for in situ hybridization to detect AVP mRNA. We found that neonatal TP treatment significantly increased the number of vasopressinergic cells in the BST over control injections. We then sought to determine the effects of testosterone metabolites, estradiol and dihydrotestosterone, given alone or in combination, on AVP expression in the BST. Rat pups were treated as described above, except that instead of testosterone, estradiol benzoate (EB), dihydrotestosterone propionate (DHTP), a combination of EB and DHTP (EB+DHTP), or vehicle was injected neonatally. Neonatal treatment with either EB or EB+DHTP increased the number of vasopressinergic cells in the BST over that of DHTP or oil treatment. However, treatment with DHTP also significantly increased the number of vasopressinergic cells over that of oil treatment. Hence, in addition to bolstering evidence that estradiol is the more potent metabolite of testosterone in causing sexual differentiation of the brain, these data provide the first example of a masculinizing effect of a nonaromatizable androgen on a sexually dimorphic neuropeptide system.     

Blockade of lordosis by androst-1,4,6-triene-3,17-dione (ATD) and tamoxifen in female hamsters primed with testosterone propionate

Normal female hamsters display lordosis after testosterone propionate (TP) plus progesterone (P) treatments. Such effect is probably mediated through aromatization of testosterone (T) into estradiol. If so, then an aromatase inhibitor (ATD) or an estrogen antagonist (tamoxifen, TAM) should be able to block the activational effect of T on lordosis. To test this hypothesis, 48 ovariectomized female hamsters were assigned into six groups which, according to treatments received, were ATD + TP, TAM + TP, OIL + TP, ATD + EB (estradiol benzoate), TAM + EB, and OIL + EB groups. The groups received assigned treatments for 2 days and were injected with P on the third day. Five minutes of behavior test was conducted 4 hr after P injection. The OIL + TP, OIL + EB, and ATD + EB groups all had averaged total lordosis duration (TLD) longer than 200 sec. The TLD of the TAM + EB group was only 117 sec. The ATD + TP and TAM + TP groups showed almost no lordosis. The results showed that the estrogen antagonist (TAM) impaired lordosis no matter whether the animals were primed with TP or EB, but the aromatase inhibitor (ATD) blocked lordosis only in TP primed females. It is concluded that the aromatization of T to estrogen is required for testosterone activation of lordosis in female hamsters.     

Decidualization in rats given 5alpha-dihydrotestosterone or its propionate neonatally.

In Experiment 1, female rats were given a single subcutaneous injection of 1.25 mg 5alpha-dihydrotestosterone (DHT) or its propionate (DHTP) on day 5 of postnatal life. All of them showed regular estrous cycles as adults like untreated control animals. At about 60 days of age, the rats were ovariectomized and given 7 daily injections of 2 mg progesterone (P) plus 0.2 mug estradiol-17beta (ED). Uterine trauma applied on the 4th day of P-ED injections resulted in well developed deciduomata in all animals by the day after the last injection. This made a sharp contrast to the failure of female rats receiving testosterone propionate (TP) neonatally to give a positive response under similar experimental conditions (Takewaki and Ohta, 1974). The mean weight of traumatized horns was significantly larger in DHTP-treated rats (but not in DHT-treated rats) than in controls. In Experiment 2, rats were ovariectomized on day 4 and given a dose of 1.25 mg DHT or DHTP on day 5. Controls were ovariectomized on day 4 but not given any steroid on the next day. A series of 7 daily injections of 2 mg P plus 0.2 mug ED was started at about 60 days of age, after the animals had received 3 daily injections of 0.2 mug ED or 30 daily injections of 0.1 mug ED. Incidence of deciduomata following uterine traumatization was markedly lowered only in animals treated with DHTP neonatally and given 0.1 mug ED for 30 days as adults, no significant differences being found in both incidence and size of deciduomata among the other groups. It was suggested that the effects of neonatal steroid administration on uterine responsiveness in adulthood are specific to the steroid. The previous conclusion that persistent estrus in androgen-sterilized rats plays a part in the reduction of uterine responsiveness was confirmed. An exposure of rats to estrogen for a prolonged postpuberal period was without effect, unless the animals had received enough androgen neonatally.     

Induction of male mating behavior in androgen-insensitive (tfm) and Normal (King-Holtzman) male rats: effect of testosterone propionate, estradiol benzoate, and dihydrotestosterone

Castrated androgen-insensitive rats exhibited mounting and intromission patterns in response to testosterone propionate (TP), estradiol benzoate (EB), or EB combined with dihydrotestosterone (DHT) treatment in adulthood. Treatment with DHT alone was ineffective in stimulating male mating behavior in the mutant rats. Since androgen-insensitive rats, like normal males, have the potential to show mounting behavior following hormone treatment in adulthood, the neural substrate underlying this behavior must be masculinized during development. The effectiveness of gonadal hormones in activating the entire copulatory sequence in castrated littermate males (King-Holtzman) was also examined. TP treatment induced mating behavior in the control rats. DHT also stimulated the complete copulatory pattern, although it was not as effective as TP. The administration of EB, however, did not induce ejaculation in control rats. These results do not support the hypothesis that the activation of male mating behavior by testosterone requires its metabolite estrogen (aromatization hypothesis).     

Estrogenic contributions to sexual differentiation in the female guinea pig: influences of diethylstilbestrol and tamoxifen on neural, behavioral, and ovarian development

We administered the synthetic estrogen, diethylstilbestrol (DES), or the antiestrogen, tamoxifen, to pregnant guinea pigs and observed the consequences for sexual differentiation of their female offspring. Hormones were administered during the period when treatment of fetuses with testosterone influences the development of sex-related traits (approximately Days 30 to 65 of gestation). Ovarian function, masculine and feminine sexual behavior, and the structure of a sexually dimorphic neural region in the preoptic area were assessed in adulthood in hormone-exposed animals and in oil-treated and untreated controls. Prenatal exposure to DES dipropionate (DESDP) caused masculinization and defeminization. DESDP-treated females mounted more than control females, both without hormonal stimulation and when given testosterone propionate (TP) as adults. The sexually dimorphic neural region was also masculinized in these females. In regard to defeminization, they showed delayed vaginal opening, impaired progesterone (P) production, an absence of corpora lutea, and impaired lordosis and mounting responses to estradiol benzoate (EB) and P. Prenatal treatment with tamoxifen produced a complicated pattern of results. Tamoxifen-exposed females evidenced less masculine-typical behavior, showing diminished mounting without hormonal stimulation and in response to TP. However, they also showed delayed vaginal opening, enhanced P production, and impaired mounting in response to EB and P. Their lordosis behavior and the volume of the sexually dimorphic neural region were unaffected. These results suggest that estrogens play a substantial role in sexual differentiation in the guinea pig. High levels of estrogen promote masculine-typical development, and unusually low levels may impair some aspects of both masculine-typical and feminine-typical development.     

Effects of gonadal steroids on agonistic behavior of female Peromyscus leucopus

The role of gonadal hormones in modifying agonistic behavior of female P. leucopus was examined by means of ovariectomy and treatment with estradiol benzoate (EB), progesterone (P), or testosterone propionate (TP). Aggression was lower in diestrous females than in proestrous females, and was eliminated by ovariectomy. Submissive behavior increased following ovariectomy; surgery had no effect on investigative behavior. Administration of EB had no effect on aggressive or submissive behavior, but higher dosages caused an increase in investigative and sexual behavior. Higher dosages of P increased aggression; P had no effect on submissive or investigative behavior. An increase in aggression also resulted from administration of high levels of TP. TP also caused an increase in investigative behavior, and had no effect on submissive behavior. These results may be due to direct effects of the administered hormones on behavior or to indirect effects such as a stimulation of prolactin secretion or alteration of adrenal function.     

Sexual behavior and penile reflexes of neonatally castrated male rats treated in infancy with estrogen and dihydrotestosterone

Male rats castrated neonatally and treated with a combination of 0.5 μg estradiol benzoate (EB) plus 50μg dihydrotestosterone propionate (DHTP) for the next 14 days displayed normal sexual behavior when injected with testosterone propionate (TP) in adulthood. Neither EB nor DHTP alone had this developmental effect inasmuch as only 20–25% of the neonatal castrates treated with just 0.1, 0.5, or 10 μg EB, or 50 μg DHTP, displayed ejaculatory responses. The periodic application of mildly painful electric shock, which has been previously shown to markedly facilitate ejaculatory responding in normal male rats, failed to improve sexual performance in these latter subjects. This was true even of the castrates treated neonatally with DHTP which frequently intromitted. Castrates treated with EB or DHTP alone neonatally were subjected to spinal transection (after testing of sexual behavior) for examination of penile reflexes. Those treated with DHTP showed normal reflexes, characterized by numerous erections and flips, indicating the presumably nonaromatizable DHTP has developmental effects on penile reflexes similar to those of testosterone. Subjects treated with EB, including four animals that had ejaculated at least once, displayed very few, if any, erections on reflex tests and no flips. These results show that sometimes intromissive and ejaculatory patterns can occur even though the animal appears to have little or no capacity for penile reflexes.     

Effects of an estrogen antagonist,MER-25, on mounting behavior and lordosis behavior in the female rat

Four daily injections of 20 mg ethamoxytriphetol, MER-25, to intact female rats with regular 4-day estrous cycles inhibited lordosis behavior, but had no inhibitory effect on mounting behavior. Ten mg/day of MER-25 for 9 days partially antagonized the stimulatory effect of 2 μg/day of estradiol benzoate on lordosis behavior in ovariectomized female rats, but had no inhibitory effect upon mounting behavior. MER-25 (10 mg/day for 9 days) stimulated the display of mounting behavior in ovariectomized female rats. No effects of MER-25 treatment (10 mg for 10 days) comparable to those of testosterone propionate (10, 50, or 250 μg for 10 days) on testicular, seminal vesicle, or ventral prostate weights of intact male rats or on seminal vesicle or ventral prostate weights of castrated male rats were observed. The results show that MER-25 acts differently upon various estrogen sensitive behaviors in the female rat.     

Differential effects of the anti-estrogen MER-25 and of three 5alpha-reduced androgens on mounting and lordosis behavior in the rat.

Four experiments were performed in order to evaluate further the hypothesis that androgen must be aromatized to estrogen for the activation of masculine sexual behavior in the male rat. In Experiment 1 it was found that the anti-estrogen MER-25 failed to disrupt mounting behavior in castrated males which simultaneously received testosterone propionate (TP). However, in Experiment 2 it was found that MER-25 as weil as 3β-androstanediol effectively activated masculine behavior in castrated males treated simultaneously with dihydrotestosterone propionate. Both MER-25 and 3β-androstanediol had previously been shown to display an affinity for cytoplasmic estradiol-17β receptors present in male rat anterior hypothalamus. In Experiments 3 and 4, performed with ovariectomized females, it was found that whereas MER-25 antagonized the stimulatory effect of estradiol benzoate (EB) on lordosis behavior, 3β-androstanediol did not. In addition, 5α-dihydrotestosterone and 3α-androstanediol, two compounds which had previously been shown to have almost no affinity for estradiol-17β receptors in the hypothalamus, both inhibited the stimulatory effect of EB on lordosis. It is concluded that the fact that anti-estrogens suppress lordosis induced in females with either EB or TP, but fail to disrupt TP-induced mounting behavior in male rats does not argue against the aromatization hypothesis for masculine sexual behavior.