Correlation between sexual maturation and induction of persistent estrus by continuous illumination.

The rats brought up under continuous illumination from the neonatal day persist in the rhythmic estrous cycle for long time and do not easily reveal so-called continuous vaginal estrus. On the contrary, immature rats at about 30 days of age show the persistent estrus soon after exposure to continuous illumination as well as adult rats. The purpose of this paper was to examine the relation of the age placed under continuous illumination and the time required to reach the persistent estrus. As the results, it was found that the younger rats need proportionally the longer time to reach the persistent estrus; the rats which were transferred into continuously illuminated environment at 0 (group A), 5 (group B), 10 (group C), 15-16 (group D) and 30 (group E) days of age required a mean time of 132.1 +/- 8.4, 126.8 +/- 8.9, 89.7 +/- 6.2, 91.5 +/- 3.8 and 70.3 +/- 3.0 days after birth to reach the persistent estrus, respectively. In the present data, a significant difference was seen between group B and C. Though all rats were killed after the same length of persistently estrous periods, actual endocrine level appeared to differ among the groups; the rats required longer time to reach the persistent estrus had smaller ovaries and adrenals.     

Changes in hypothalamic serotonin concentration following manipulations on the gonadotrophic gonadal axis of the constant estrous anovulatory (CEA) rat.

The effect of pinealectomy or of injection of luteinizing hormone (LH) containing pituitary extract on the serotonin concentration of the hypothalamus was comparatively investigated in female rats with three different types of constant estrous anovulatory (CEA) syndrome. CEA syndrome was provoked by frontal hypothalamic deafferentation (FHD), by neonatal androgen treatment (NA) and by exposure of continuous illumination (light induced constant estrous; LCE syndrome). Pinealectomy caused an increase in hypothalamic serotonin concentration in the FHD rat, but failed to increase it in the NA and LCE group. The injection of crude anterior pituitary extract, however, provoked significant elevation of the hypothalamic serotonin concentration equally in all three types of CEA syndrome. Ovariectomy in itself failed to cause any significant change in the serotonin level of the brain. However, pinealectomy or the injection of LH containing pituitary extract proved to be effective also in the ovariectomized CEA animals. It is concluded that the increase in the brain-serotonin concentration of CEA rats, observed after pinealectomy or after injection of LH containing crude pituitary estract, is running through a hypothalamo-pituitary mechanism rather than through the hormone secreting activity of the ovaries.     

Delayed persistent estrus induced by continuous lighting after inadequate acclimation in rats.

While the normal estrous cycle of adequately acclimated female rats was replaced by a persistent estrus (PE) under continuous lighting, the onset of PE was delayed following several irregular cycles without acclimation or after acclimation for one week, suggesting that transportation induces a significant critical stress.     

Conditions for establishment of reflex ovulation in light estrous rats.

Continual anovulatory state associating with persistent vaginal cornification (light estrus) was induced by placing 4-day cycling rats under continuous lighting (LL). Uterine cervical stimulation was applied at arbitrary solar hours to light estrous rats showing continual vaginal estrus for more than 2 weeks. The ovulation was induced between 14 and 16 hr after the stimulation dissociating entirely with solar hours. Injection of anti-LHRH serum 5 min after the stimulation but not later than 20 min blocked this ovulation. Ovulation thus induced was always followed by pseudopregnancy with continual leucocytic vaginal smear lasting 10.70 days. The change in concentrations of peripheral serum progesterone during this period was almost similar to that of normal pseudopregnancy except extremely low levels observed at the start and end. Effectiveness of the cervical stimulation for induction of ovulation in light estrous rats was related to not only the duration of light estrus but also the time after transfer to LL, suggesting that the neural mechanism of ovulation in light estrous rats shifted from that of the spontaneous to reflex ovulators due to the extinction of environmental photic cue.     

Effect of long-term pinealectomy on serum LH levels and on cerebral serotonin metabolism in adult male rats

Two months after pinealectomy carried out in adult male rats previously exposed to a long photoperiod (L:D 14:10) during 10 weeks, the weight of seminal vesicles significantly increased in comparison with Sham-operated animals. Nevertheless, the weights of testes and ventral prostate did not change significantly. The serum LH concentration increased significantly by pinealectomy. The levels of serotonin and 5-hydroxiindolacetic acid significantly decreased in the hypothalamus and midbrain after pinealectomy.     

Prevention of continuous light-induced anovulation in rats by early exposure to continuous light

Continuous illumination (LL) beginning at 22 days of age caused precocious puberty followed by persistent estrus with anovulation in female offspring originating from mother rats exposed to a 14L:10D light-dark cycle prior to and during pregnancy. However, LL had no deleterious effect on reproductive cycles of offspring reared in LL and originating from mothers exposed to LL prior to and during pregnancy. These rats had a normal onset of puberty in LL, a normal 4-day estrous cycle, a periodic rise of plasma estrogen prior to the periodic appearance of the preovulatory luteinizing hormone (LH) surge, and spontaneous ovulation in LL continued until at least 300 days of age. Also, the female offspring of these rats showed a similar resistance to the deleterious effects of LL on cyclic ovulation. These results support the following interpretation: 1) offspring from mother rats exposed to LL prior to and during pregnancy become insensitive to the deleterious effects of LL on cyclic ovulation, 2) neural elements controlling cyclic release of LH are not totally photoperiod (14L:10D)-dependent, and 3) in the absence of daily 14L:10D signals, an endogenous clock, possibly timed by daily laboratory signals (temperature, noise, taking of vaginal smears), may provide time cues for cyclic LH release.     

Electron microscopy of two types of gonadotrophs in the anterior pituitary glands of persistent estrous and diestrous rats

Summary Persistent estrus and diestrus was produced in rats by the administration of estrone for either 5 days or 30 days, respectively, immediately after birth. Female rats without any treatment were used for control. After these rats grew up, the anterior pituitaries were examined by electron microscopy. The identification criteria for two types of gonadotrophs, FSH-and LH-cells, proposed by Barnes were adopted. In the persistent estrous rats, FSH-gonadotrophs were almost normal, but LH-gonadotrophs were filled with an abundance of secretory granules which were probably suppressed in discharge. On the other hand, in the persistent diestrous rats, FSH-cells were few in number and strongly atrophic, containing a few secretory granules, while LH-cells were almost normal or rather slightly activated. These electron microscopic findings well coincide with the results of light microscopy of ovaries, which suggested that in the persistent estrous rats FSH secretion might be almost normal but the secretion of LH might be inhibited, while in the persistent diestrous rats FSH secretion might be almost totally abolished but LH might be moderately secreted. From these findings, identification of FSH-and LH-gonadotrophs in the anterior pituitary of the rat well coincides with that proposed by Barnes in mice.     

Effects of acute and chronic restraint and estrus cycle on pituitary-adrenal function in the rat

Female Long-Evans hooded rats with 5-day estrus cycles were subjected to 4 hr of continuous restraint for either 1 or 20 days. On the last day of the stress regimen, plasma and adrenal corticosterone concentrations were determined and classified according to the stage of the estrous cycle. The results indicated that acute stress produced greater plasma corticosterone concentrations than controls only during estrus, whereas in response to chronic stress significant stress-induced increments were observed during estrus and proestrus. The results suggest that the estrous cycle influences the magnitude of the stress-induced increments for both acute and chronic stress. In addition, the pituitary-adrenal system did not show habituation to repeated administration of this stress, but sensitization was observed during proestrus.     

The effect of luteinizing hormone, human chorionic gonadotropin and adrenocorticotropic hormone on puberty in gilts reared in confinement

Three trials were conducted to determine the effect of human chorionic gonadotropin (HCG), luteinizing hormone (LH) and adrenocorticotrophic hormone (ACTH) on the incidence of estrus in gilts which were reared in confinement, relocated and exposed to a boar. In trial 1, 33 gilts were given saline or 250 IU HCG at an average age of 191 days and then relocated and observed for estrus twice daily for 10 days. Treatment with HCG did not increase the proportion of gilts that exhibited estrus. In trial 2, 42 gilts were relocated at an average age of 200 days. The gilts were assigned to three treatment groups and injected with saline, 68 mug LH or 1 mg LH. After 10 days of estrous detection, a laparoscopic examination of the ovaries was conducted on all gilts failing to exhibit estrus. In groups 1 to 3, the proportions of gilts exhibiting estrus or ovulating during the 10 days after treatment were 13 of 21, 6 of 10, and 5 of 11, respectively. In trial 3, 12 gilts were relocated to pasture lots, given saline or 80 IU ACTH twice daily for 2 days and checked for estrus for 14 days. The proportions of gilts that exhibited estrus after the administration of saline or ACTH were 4 of 6 and 6 of 6, respectively. The results indicate that the incidence of estrus in gilts reared in confinement, relocated and exposed to a boar was not affected by pre-treatment with exogenous HCG, LH or ACTH.     

Effects of neonatal treatment with phytoestrogens, genistein and daidzein, on sex difference in female rat brain function: estrous cycle and lordosis

It is well known that neonatal exposure to estrogen induces masculinization or defeminization of the brain. In this study, the effects of neonatal treatment with two kinds of soybean isoflavone aglycone, genistein (GS) and daidzein (DZ), on the estrous cycle and lordosis behavior were investigated. Female rats were injected subcutaneously with 1 mg GS, 1 mg DZ, 100 microg estradiol (E2), or oil daily for 5 days from birth. As a result, vaginal opening was advanced in GS- or E2-treated females. A vaginal smear check indicated that oil- or DZ-treated females showed a constant 4- or 5-day estrous cycle, whereas GS- or E2-treated rats showed a persistent or prolonged estrus. Ovariectomy was performed in all females at 60 days of age. The ovaries in the GS- or E2-treated groups were smaller than those in the oil- and DZ-treated groups and contained no corpora lutea. In the DZ group, although corpora lutea were seen, ovaries were smaller than that of control females. Behavioral tests were carried out after implantation of E2-tubes. All of the oil- or DZ-treated females showed lordosis with a high lordosis quotient (LQ). On the other hand, as male rats, LQs were extremely low in the E2-treated group, when compared to the oil-treated group. In the GS-treated group, the mean LQ was lower than that in the oil-treated group, but higher than those in the E2-treated female or male groups. These results suggest that genistein acts as an estrogen in the sexual differentiation of the brain and causes defeminization of the brain in regulating lordosis and the estrous cycle in rats. In addition, neonatal daidzein also has some influence on ovarian function.     

Sex and estrous cycle differences in the behavioral effects of high-strength static magnetic fields: role of ovarian steroids

Advances in magnetic resonance imaging are driving the development of higher-resolution machines equipped with high-strength static magnetic fields (MFs). The behavioral effects of high-strength MFs are largely uncharacterized, although in male rats, exposure to 7 T or above induces locomotor circling and leads to a conditioned taste avoidance (CTA) if paired with a novel taste. Here, the effects of MFs on male and female rats were compared to determine whether there are sex differences in behavioral responses and whether these can be explained by ovarian steroid status. Rats were given 10-min access to a novel saccharin solution and then restrained within a 14-T magnet for 30 min. Locomotor activity after exposure was scored for circling and rearing. CTA extinction was measured with two-bottle preference tests. In experiment 1, males were compared with females across the estrous cycle after a single MF exposure. Females circled more and acquired a more persistent CTA than males; circling was highest on the day of estrus. In experiment 2, the effects of three MF exposures were compared among intact rats, ovariectomized females, and ovariectomized females with steroid replacement. Compared with intact rats, ovariectomy increased circling; estrogen replacement blocked the increase. Males acquired a stronger initial CTA but extinguished faster than intact or ovariectomized females. Thus the locomotor circling induced by MF exposure was increased in females and modulated by ovarian steroids across the estrous cycle and by hormone replacement. Furthermore, female rats acquired a more persistent CTA than male rats, which was not dependent on estrous phase or endogenous ovarian steroids.     

Physical properties of bovine cervical mucus during normal and induced (progesterone and/or PGF2alpha) estrus

Ninety two Friesian cows were used to determine physical properties of cervical mucus collected during normal estrus and estrus induced. Estrus was induced using either progesterone (P4) releasing intravaginal devices (PRID) and/or prostaglandin F2alpha (PGF2alpha). The animals were assigned to 4 groups (no treatment, a PRID for 12 days plus an injection of 1000 IU PMSG at the removal of the PRID, a double injection of 3 mL PGF2alpha 11 days apart, and a PRID for 7 days plus an injection of PGF2alpha 24 h before the removal of PRID). A number of cows with normal estrus exhibited three consecutive estrus cycles after calving. Cows that had not shown estrus for three months after calving had their reproductive system palpated twice at 10-day intervals, to determine their ovarian activity. Then PRID and/or PGF2alpha was administered to cows that were found to have a palpable corpus luteum in one of two palpations (cycling cows). The cows of the three induced estrous groups were artificially inseminated (AI) twice, while those with normal estrus received only a single AI. Cervical mucus samples were collected from all cows 5 to 30 min before the first AI. Additionally, samples of cervical mucus were collected from 20 cows at their first estrus after the induced estrus. The results are summarized as follows: 1) The physical properties of cervical mucus were similar in the first three normal consecutive estrus cycles after calving. 2) The physical properties of cervical mucus in normal estrus after calving were similar to those in the first estrus after an induced estrus. 3) The pH values for normal estrus were similar to those for induced estrus. 4) Viscosity of cervical mucus in the normal estrous group was significantly lower than that in the induced estrus. Furthermore, significant differences were noticed among the three induced estrous groups. 5) Spinnbarkeit, crystallization and receptivity of cervical mucus (penetration test) were significantly higher in the normal estrous group than in the induced estrous groups, while no difference was detected among induced estrus groups. 6) Pregnancy rates in the normal estrus group were the same as in the induced estrus groups. 7) The percentages of cows in the induced estrous groups that produced cervical mucus with similar viscosity, spinnbarkeit and receptivity (penetration test) characteristics as the normal estrus group, was very low.     

Effect of thyroid hormone on initiation of persistent oestrus in the rat.

When rats are exposed to continuous illumination persistent oestrus and polyfollicular ovaries develop. Thyroidectomy at 24 days of age (juvenile rats) prevents the onset of persistent oestrus and polyfollicular ovaries under exposure to constant light, and irregular ovulation continues to occur. Replacement with 1.75 microng L-thyroxine in these rats produces a prolonged oestrus. However, in adult persistent-oestrous rats (90 days after exposure to continous illumination) thyroidectomy does not interrupt persistent vaginal cornification. In rats receiving 100 microng of testosterone propionate subcutaneously at 5 days of age, persistent oestrus and polyfollicular ovaries develop. Thyroidectomy at 24 days of age (juvenile) prevents the onset of persistent oestrus and the development of polyfollicular ovaries, however, ovulation is not observed. Replacement treatment with 1.75 microng L-thyroxine in these rats produces a prolonged oestrus and polyfollicular ovaries. However, in adult persistent oestrus rats, thyroidectomy at 130 days does not interrup persistent vaginal cornification. From these facts, it may be inferred that circulation of a physiological level of thyroid hormone in juvenile rats is necessary for the development of oestrogen binding receptors in the hypothalamus. Therefore, a hypothyroid state during the juvenile stage interferes with the development and maturation of hypothalamic controlled pituitary ovarian function.     

The beta1-adrenoceptor site activated by CGP12177 varies in behavior according to the estrous cycle phase and stress

The aim of this work was to assess whether stress and estrous cycle phases affected the beta1-adrenoceptor (beta1-AR) site activated by CGP12177 in the right atria of rats. The chronotropic response to CGP12177 in the absence or presence of antagonists was determined in atria from rats submitted to one daily foot-shock session for 3 consecutive days. Blood was collected for hormonal assays. The pD2 for CGP12177 in atria from females was lower than in atria from males and was unaltered by stress or the estrous cycle. Propranolol (200 nM) or CGP20712A (3 microM) shifted the concentration-response curves to CGP12177 to the right in control and stressed estrus or control diestrus rats. Atria from stressed diestrus rats were resistant to blockade by propranolol or CGP20712A, indicating that the effect of beta-adrenoceptor antagonists on the response to CGP12177 is influenced by estrous cycle phases. The stress-induced increase in serum corticosterone levels was independent of the estrous cycle or gender, but the estradiol/progesterone ratio was affected differently in the two groups of female rats. In the diestrus group, serum estradiol levels decreased after the first foot-shock session and remained low until the day of sacrifice, whereas in the estrus group the serum levels of estradiol did not decrease after stress and peaked on the second day, which corresponded to proestrus. These data do not indicate whether there is a direct or indirect effect of stress hormones and (or) sex steroids on cardiac beta1-AR sensitivity. However, they do show that the classic and low-affinity binding sites of the beta1-AR are independently regulated and that the beta1-AR atypical site affinity for antagonists depends on the estrous cycle.     

The efficacy of estrus detection and fertility following synchronization with PGF2a or synchro - mate- B in Zebu cattle

Two experiments were carried out to assess the efficacy of estrus detection and fertility in Zebu cattle after synchronization with prostaglandin F2a or a progestagen. The first experiment compared estrus detection rates and fertility following insemination in 42 cows previously synchronized with either 25 mg of PGF2a or with a 6 mg of Norgestomet implant plus 5 mg i.m. of estradiol valerate (SMB). Differences were observed in the percentage of cows detected in estrus (54 vs 95%, respectively, P < 0.05), but not in fertility at the first synchronized estrus (26 vs 15%), nor in the detection rate and fertility at the subsequent estrous period (38 v 47%). The second experiment evaluated the efficacy of estrus detection at different time intervals in 30 cows, comparing estrus synchronized with PGF2a with the subsequent estrous period. The observation periods were continuous, day and night, for 100 h both after PGF2a treatment and from Day 18 of the treatment cycle (Period 1). In addition, the animals were administered PGF2a again on Day 10 of the second cycle and observed continuously from 0600 to 1800 h, and from Day 18 of the treatment cycle (Period 2). Finally, the same treatment regimen was used except that the observation was between 0600 to 0700 h and 1800 to 1900 h (Period 3). No differences were obtained in the percentage of cows detected in estrus in the synchronized and nonsynchronized groups (average 75%); however, accuracy in the detection of estrus in Period 3 differed in the nonsynchronized and synchronized estrus groups by 40% (P < 0.05) compared with the other two, more intense observation periods.     

The role of ovarian sympathetic innervation in the control of estrous responsiveness in the rat

This study examined the contribution of the superior ovarian nerve (SON) to estrous responsiveness and ovarian function in cycling rats. Section of the SON was carried out at 1100 on proestrus, and lordotic responsiveness was measured at 1500, 1700, and 2100 on that day and at 0900, 1200, and 1500 on the day of estrus. SON section decreased lordosis intensity significantly at 1500 and 1700 on proestrus and at 0900 on estrus. Pacing of sexual contacts with males was decreased at 2100 in nerve-sectioned (Nervx) animals when compared with sham-operated controls (SHAM). Serum progesterone (P) concentrations were significantly lower in Nervx animals than in Sham animals 30 min after surgery, but were not different between groups at 4.5 hr. Serum estradiol (E2) concentrations did not differ between groups at either time. In addition, Nervx and Sham groups did not differ on measures of pregnancy/pseudopregnancy initiation or on measures of ovarian function 10 days after surgery. These data suggest that the integrity of the SON is necessary for the display of full estrous responsiveness in cycling rats, and suggest that the acute decreases in serum P occurring as a consequence of SON section may be responsible for the deficits seen in Nervx animals.     

Effects of complete hypothalamic deafferentation on the estrous phase of follicle-stimulating hormone release in the cyclic rat

We investigated whether neural afferents to the medial basal hypothalamus play an acute role in the estrous phase of FSH release in the 4-day cyclic rat. A cannula was inserted into the right atrium of the heart under brief ether anesthesia during the early afternoon of proestrus for subsequent blood collections and injection of LHRH. In some of the rats, the medial basal hypothalamus was surgically isolated from the rest of the brain with a small knife under brief ether anesthesia between 2000 h and 2130 h of proestrus. Control groups consisted of naive rats which were not treated during the night of proestrus and sham-operated animals in which the knife was lowered to the corpus callosum between 2000 h and 2130 h or proestrus. Rats were bled at 2200 h of proestrus and at 0200 h, 0600 h and 1000 h of estrus for radioimmunoassay of plasma FSH and LH. The plasma FSH levels in all 3 groups between 2200 h of proestrus and 1000 h of estrus were elevated above levels observed in other cannulated rats bled to the onset of the proestrous phase of FSH release at 1400 h of proestrus. There were no statistically significant differences in plasma FSH or LH concentrations at any of the time periods between the 3 groups of serially bled rats. The deafferentation procedure did not appear to impair the pituitary gland's ability to secret gonadotrophins as injection of 50 ng of LHRH after the bleeding at 1000 h of estrus caused substantial elevations in plasma FSH and LH concentrations which were not different between the 3 groups. The results suggest that neural afferents to the medial basal hypothalamus play no acute role in the estrous phase of FSH release in the cyclic rat.     

Physiological pineal effects on female reproductive function of laboratory rats: prenatal development of pups, litter size and estrous cycle in middle age

The present study investigates whether and how the pineal or its hormone melatonin influences female reproductive functions, namely the litter size, prenatal development of offsprings, and estrous cyclicity, especially its age-related cessation in a non-seasonal breeder, the laboratory rat. Wistar rats were maintained under a 24 h light-dark (12Lratio12D) cycle. Female rats were divided into 3 groups: non-operated (NO), sham-operated (SX), and pinealectomized (PX). Surgeries were performed in 35-40 day-old females. Starting at an age between 70 days and 7 months, female rats of all 3 groups were repeatedly mated with intact males. PX mothers more frequently delivered pups with malformations (e.g., taillessness, hydronephrosis, 7 out of 1263 pups) than control rats (0/1323; p<0.007). In the first delivery at 3 months of age, but not at later ages, PX mothers delivered more pups of lower body weight than control animals (p<0.001). Examination of vaginal smears showed that almost all female rats of the NO, SX, and PX groups had 4-day estrous cyclicity when they were young-between 60 days and 5 months of age. At an age of 17 to 18 months, most female rats of the NO and SX groups showed irregular, continuously diestrous or pseudopregnancy-like patterns, and 4-day estrous cyclicity was found in only 10% of the NO or SX animals. In contrast, about 50% of the PX rats showed 4-day estrous cyclicity at this older age (p< 0.001). Melatonin, when added to drinking water (0.4 mg/L) for 16 days during the dark phase increased the frequency of diestrous phase, except in continuously diestrous rats and very few others. This melatonin effect was strong in PX rats but relatively weak in SX rats. In conclusion, the pineal hormone appears to influence various reproductive functions and developmental processes, especially pregnancy and the timing of reproductive aging in rats. The effects of pinealectomy are more prominent at an age of 60 to 80 days (i.e., shortly after puberty) and at the beginning of the cessation of cycles in middle-aged females.     

Are endogenous sex hormones related to DNA damage in paradoxically sleep-deprived female rats?

The aim of this investigation was to evaluate overall DNA damage induced by experimental paradoxical sleep deprivation (PSD) in estrous-cycling and ovariectomized female rats to examine possible hormonal involvement during DNA damage. Intact rats in different phases of the estrous cycle (proestrus, estrus, and diestrus) or ovariectomized female Wistar rats were subjected to PSD by the single platform technique for 96 h or were maintained for the equivalent period as controls in home-cages. After this period, peripheral blood and tissues (brain, liver, and heart) were collected to evaluate genetic damage using the single cell gel (comet) assay. The results showed that PSD caused extensive genotoxic effects in brain cells, as evident by increased DNA migration rates in rats exposed to PSD for 96 h when compared to negative control. This was observed for all phases of the estrous cycle indistinctly. In ovariectomized rats, PSD also led to DNA damage in brain cells. No significant statistically differences were detected in peripheral blood, the liver or heart for all groups analyzed. In conclusion, our data are consistent with the notion that genetic damage in the form of DNA breakage in brain cells induced by sleep deprivation overrides the effects related to endogenous female sex hormones.     

Increases in the basal secretion rate of follicle-stimulating hormone (FSH) accompany age-associated changes in serum FSH levels on estrus

This laboratory has recently reported that by 5-6 months of age, alterations in the secretion and production of follicle-stimulating hormone (FSH) occur in virgin female rats which precedes the age-related disruption of estrous cycles and attenuation of preovulatory gonadotropin surges. Specifically, circulating immunoreactive FSH levels are higher on estrus in rats 5 months and older compared to levels measured in 2- to 3-month-old rats. Therefore, the present study was conducted to explore a possible mechanism for this age-related increase in FSH levels. At 1400 hr on proestrus, estrus and diestrus-1, groups (n = 6-12 rats/group) of 3- and 7-month-old, cyclic rats were decapitated, trunk blood was collected, and anterior pituitary glands were bisected and placed in incubation flasks containing 1 ml media (medium 199). Following a 30-min preincubation period, hemipituitary fragments were incubated for an additional 2 hr. Media and serum FSH levels were quantified by RIA. Levels of FSH were twofold higher in the serum of 7-month-old rats than 3-month-old rats on estrus. Similarly, the basal secretion rate (BSR) of FSH (expressed as ng FSH/ml/2 hr) was significantly (P less than 0.05) higher from incubated hemipituitary fragments of 7-month-old estrous rats than from fragments obtained from younger estrous rats (7 month: 1637 ng/ml/2 hr vs 3 months: 1253 ng/ml/2 hr). Neither the serum FSH levels nor the BSR of FSH differed between age groups on proestrus or diestrus-1. These results show that age-associated increases in circulating FSH levels on estrus may be attributed to an enhanced basal secretion of FSH from the pituitary gland.