Sexual behavior of mares

The mare is seasonally polyestrus, having an anovulatory period during the short light days of late fall and early winter, and beginning to ovulate as the days become longer during the winter. The complete estrus cycle is typically about 3 weeks, with 5 to 7 days of estrus and approximately 2 weeks of diestrus. When a mare lives within the natural social structure of the horse, i.e. a family band with several adult mares and one or more stallions, estrus is characterized by repeatedly approaching the stallion, frequent urination, deviating the tail away from the perineum, and standing still with the hind limbs spread apart. Diestrus is characterized by avoidance of an approaching stallion, and aggression toward the stallion, such as squealing, striking, and kicking, if he persists in attempting to court the diestrus mare. However, mares and stallions with long-term social relationships will often rest together, graze together and groom each other, all without sexual interactions. Hormonally, estrous behavior in the mare is initiated by estradiol that is secreted by the follicle, while estrous behavior is suppressed by progesterone, secreted by the corpus luteum. Mares are unusual among the ungulates in that they periodically exhibit estrous behavior during the anovulatory period. This is probably due to the release of estrogenic steroids secreted by the adrenal cortex. The display of sexual behavior by the mare throughout the year is thought to facilitate maintenance of the horse's social structure, in which the male remains with a group of females year round, in contrast with most ungulates in which the females and males only come together during the mating season.     

Induction of male sex behavior in pony mares with testosterone propionate

Two pony mares were administered 150 mg of testosterone propionate every other day for 20 days (ten injections) and every ten days there-after. An additional two mares and one stallion were not treated and served as controls. Testosterone propionate was dissolved in absolute ethanol and administered subcutaneously. Sex behavior tests were conducted 26 and 40 days after the first injection. Control mares exhibited very little male sex behavior. Both testosterone propionatetreated mares, however, exhibited mounting, sniffing, flehmen, biting and vocalization behavior in the presence of an estrous mare. The testosterone propionate-treated mares mounted and bit estrous mares more frequently than the stallion but exhibited less sniffing, flehmen and vocalization behavior in the presence of an estrous mare than the stallion. Testosterone propionate-treated mares and the stallion mounted an estrous mare 23.3 +/- 9.7 seconds and 172.5 +/- 22.5 seconds, respectively, after being introduced into the pen. Mares in estrus were mounted by the testosterone propionate-treated mares and the stallion an average of 4.0 +/- 1.3 and 1.0 +/- 0 times, respectively, during a ten-minute test. None of the non-estrous mares was ever mounted by the testosterone propionate-treated mares. In summary, testosterone propionate induced male sex behavior in intact mares and the testosterone propionate-treated mares effectively detected estrous mares.     

Influence of immunization against GnRH on reproductive cyclicity and estrous behavior in the mare

The aim of this study was to evaluate the effect of active immunization against GnRH on ovarian activity, plasma progesterone and estradiol concentrations and on estrous behavior in adult mares. Eighteen cyclic mares were randomly divided into a treatment and control group. Nine mares were immunized twice with 2 mL (400 microg GnRH-protein conjugate) of a GnRH-vaccine (Improvac, CSL Limited, Australia) administered intramuscularly, 4 weeks apart. Control mares received the same amount of saline solution. Ovaries and uterus of all mares were examined weekly by ultrasonography from 3 weeks before to 60 weeks after first immunization. Thereafter, vaccinated mares were evaluated monthly until 100 weeks after first vaccination. In addition, mares were teased with a stallion for assessment of estrous behavior and blood was collected for progesterone, estradiol-17beta and GnRH antibody titer determination. Results demonstrate that vaccination against GnRH significantly (P<0.05) influenced all parameters, except estradiol-17beta concentration. All vaccinated mares ceased reproductive cyclicity (plasma progesterone <1 ng/mL, follicles <3 cm) within 8 weeks after the first injection and ovarian activity remained suppressed for a minimum of 23 weeks. Five mares resumed cyclicity (follicles >3 cm, progesterone >1 ng/mL) while three mares showed only follicular activity (follicles >3 cm) and one mare remained completely suppressed for the entire duration of the study. In spite of ovarian suppression, four mares expressed sporadic and one mare continuous estrous behavior. In conclusion, reproductive cyclicity in adult mares can be successfully suppressed by immunization against GnRH but the timing of resumption of cyclicity is highly variable and estrous behavior may occur in spite of ovarian suppression.     

Sociosexual behavior and the ovulatory cycle of ponies (Equus caballus) observed in harem groups

Observations of sociosexual behavior of adult ponies, made on two harem groups (each comprised of one vasectomized male and three females), were correlated with follicular development and ovulation for a total of 15 cycles (minimum of 2 cycles per female). Mean cycle length (interovulatory interval) was found to be 19.7 days, with behavioral estrus lasting 7–8 days (5.5 days preovulatory; 2.3 days postovulatory). Estrous females typically showed increased frequencies of approaching and following the stallion, urinating, presenting, clitoral winking, and tail raising. Approaching and following the stallion appeared earlier and persisted longer than other estrous responses. Deviations from the modal estrous pattern included cycles with subestrus, continual estrus, behavioral estrus in the absence of ovulation, and displays of female mounting. Dominance tests revealed that a mare's status was unaffected by the phases of the estrous cycle. The presence of more than one estrous female affected the copulatory performance of both stallions, most notably in reduced latencies to first mount, intromission, and ejaculation, in spite of differences between the stallions in sexual vigor. Each stallion usually selected the dominant mare for copulation when there were multiple estrous females present, but mounts were not displayed exclusively to one female per test. The social testing situation made apparent the importance of use of space in sociosexual communication in this species, particularly in avoidance of the stallion by diestrous mares and standing alone or in proximity to him by estrous mares.     

Sexual behavior following introduction of a stallion into a group of mares

A rested stallion was introduced daily for 30 days into each of three herds of 20 mares. Observations of sexual and mating behavior were made for one hour. The stallion remained with a herd until another stallion was introduced the following day. Other mares were isolated from stallions and were bred by artificial insemination. The diameter or growth rate of the preovulatory follicle for the six days preceding ovulation and the length of the interovulatory interval for mares which did not become pregnant were not affected significantly by the presence of a stallion. The number of breedings per hour of observation (2.4 ±0.2) and the length of the interval from introduction of a stallion into the herd of mares to first breeding (12 ±1 min) were significantly different among stallions, but the length of the interval between breedings (17 ±2 min) was not. The mean number of breedings per stallion per hour was not affected significantly by the number of posturing (estrous) mares. The number of times that the stallion rebred the same mare when more than one mare postured during the observation hour (49%) was greater (P<0.01) than what would be expected to occur by chance (30%). The hypothesis that breeding occurs preferentially in those estrous mares that are closest to ovulation was not supported, except for significantly lower breeding activity in posturing mares on days 8 and 7 before ovulation and on days 0 (day of ovulation) and ?1 (26% bred) than on days 6 to 1 (52%).     

Effect of ACTH (tetracosactide) on steroid hormone levels in the mare. Part B: effect in ovariectomized mares (including estrous behavior)

The mare is the only non-primate species known to display estrous signs after ovariectomy and adrenal hormones have been implicated as a possible cause. Moreover, in several species, estradiol seems to have a stimulatory effect on the hypothalamic-pituitary-adrenal axis. The aim of the present study was to compare the effect of ACTH (tetracosactide) on pertinent hormones [cortisol, progesterone, androstenedione, testosterone (intact and ovariectomized mares) and estradiol (ovariectomized mares only)] in intact mares in estrus with the same mares after ovariectomy (n=5). Blood samples were collected hourly from 12:00 until 14:00 h the following day (half-hourly between 14:00 and 17:00 h) on two occasions, with saline or ACTH treatment at 14:00 h (saline treatment day or ACTH treatment day). The mares, both when intact and after ovariectomy, showed a significant increase in all measured hormones, except estradiol (not measured in intact mares), after ACTH treatment, lasting at least 3h post-treatment (P<0.001). On the saline treatment day, cortisol levels in ovariectomized mares were lower than in intact mares in the evening (18:00-23:00 h), but higher at night (24:00-05:00 h). No differences in cortisol response between mares, when intact and after ovariectomy, were found after ACTH treatment (P=0.3). Androstenedione levels were lower (P<0.001) and increased less after ACTH treatment in ovariectomized mares, as compared to when intact (P<0.05). Progesterone concentrations were lower in the ovariectomized mares at night (24:00-05:00 h) on the saline treatment day and at all times on the ACTH treatment day (P<0.05). Testosterone concentrations were lower in ovariectomized mares on both treatment days, as compared to when intact (P<0.001). It was concluded that ovariectomy affected basal cortisol pattern. Ovarian androstenedione and testosterone contributed to the basal circulating levels and, in the case of androstenedione, was stimulated by ACTH. Endogenous estradiol did not act stimulatory on adrenal gland hormone production in the mare.     

Exposure to stallion accelerates the onset of mares' cyclicity

Horses (Equus caballus) belong to the group of seasonally polyestrous mammals. Estrous cycles typically start with increasing daylight length after winter, but mares can differ greatly in the timing of onset of regular estrus cycles. Here, we test whether spatial proximity to a stallion also plays a role. Twenty-two anestrous mares were either exposed to one of two stallions (without direct physical contact) or not exposed (controls) under experimental conditions during two consecutive springs (February to April). Ovarian activity was monitored via transrectal ultrasound and stallion's direct contact time with each mare was determined three times per week for one hour each. We found that mares exposed to a stallion ovulated earlier and more often during the observational period than mares that were not exposed to stallions. Neither stallion identity nor direct contact time, mare age, body condition, size of her largest follicle at the onset of the experiment, or parasite burden significantly affected the onset of cyclicity. In conclusion, the timing of estrous cycles and cycle frequency, i.e., crucial aspects of female reproductive strategy, strongly depend on how the mares perceive their social environment. Exposing mares to the proximity of a stallion can therefore be an alternative to, for example, light programs or elaborated hormonal therapies to start the breeding season earlier and increase the number of estrous cycles in horses.     

Sexual behavior in ovariectomized and seasonally anovulatory pony mares (Equus caballus)

Ten ovariectomized (OVEX) and ten intact, but seasonally anovulatory (ANOV), pony mares were observed for sexual activity with five stallions, using a “harem group” social testing paradigm (two OVEX and two ANOV mares plus one stallion per group) for 15 consecutive daily tests lasting 20 min each. All mares in both conditions showed proceptive behavior in at least one test, all mares but one were mounted, and 14 of 20 mares received ejaculations. No statistical differences were found between the two conditions for any measure of proceptivity, copulatory activity, or days in estrus. The quality of estrus was judged to be equivalent to that displayed by periovulatory mares during their initial and terminal days of estrus, but less intense than that seen near ovulation. Mares in both groups were in estrus during approximately 60–70% of the tests and only 3 of the 20 mares were sexually refractory for more than five consecutive tests. Thus, the typical 2-week phase of sexual refractoriness seen in intact diestrous mares was absent in OVEX and ANOV mares, suggesting that the ovary plays a major role in actively suppressing estrous responses during the luteal phase of the cycle.     

Pregnancy rates and sexual behavior under pasture breeding conditions in mares

Pony mares (n=480) and 16 stallions were assigned to four herds of 60 mares and one stallion (large herds) and to 12 herds of 20 mares and one stallion (small herds). The stallions remained with the herds continuously for all of the large herds and seven of the small herds. In the five remaining small herds the stallion was put into a herd for three hours every two days for 12 observation periods. Pregnancy rates and day of ovulation were estimated by size of embryonal enlargements. Mean pregnancy rates of 51% and 54% were obtained in the small herds and 42% in the large herds during a 48-day period (equivalent to two estrous cycles). Pregnancy rates for herds with the stallion present continuously were higher (P<0.01) for the small herds than for the large herds for days 1-24 (42% versus 19%). There was no effect of herd size on number of mares becoming pregnant per herd on days 1-24, but more mares (P<0.01) became pregnant during days 25-48 in the large herds (13.2 mares per herd versus 1.8). In the herds in which the stallion was present intermittently, the number of times that the stallion rebred the same mare when more than one mare was in estrus was greater (P<0.01) than what would be expected to occur by chance (observed, 21%; expected, 11%). Repeated breeding of the same mare seemed related to the availability or activity of the mare, since such mares more frequently followed and positioned themselves in the vicinity of the stallion. Most of the interferences by a mare which involved keeping the stallion and another mare apart were directed at the mare, whereas most of the interferences during mounting were directed at the stallion (P<0.01). Mares were more likely (P<0.01) to interfere when in estrus than when in nonestrus. When interfering mares were in nonestrus, their hostility was usually directed at the stallion (92%), whereas when in estrus their interference was more frequently directed at a mare (73%, P<0.01).     

Herding and snaking by the harem stallion in domestic herds

Four herds of pony mares, each consisting of a stallion and six mares, were used to characterize the nature of herding by the stallion and the factors that induced the herding behavior. Herding behaviors were compared among four successive treatments (six mares alone, stallion added, two new mares added, and entire herd moved to a new pasture). A new treatment was initiated every 7 days and behavior was studied for 5 consecutive days (Days 1-5) for each treatment. Observations were made every 10 min during a 2-h period for each day. The extent of herding was quantitated by the mean distances between mares. The extent of snaking (herding with the head and neck extended and ears held back) was scored 0, 1, 2, or 3 (nil, minimal, intermediate, and maximal, respectively). The mean distance among the original mares on Day 1 when the mares were alone was 5.0 mare lengths and was reduced (P < 0.05) to 1.9 mare lengths when the stallion was added. The mean distance among the original mares of an established stallion/mare herd (3.8 mare lengths) was reduced (P < 0.05) on the day the herd was moved to a new pasture (1.9 mare lengths), similar to the effect of the introduction of the stallion. Scores for the extent of snaking, as well as the extent of herding, were highest (P < 0.05) on Day 1 when the stallion was added or the stallion/mare herd was moved to a new pasture. The extent of herding and snaking decreased (P < 0.05) by Day 2 and was seen only occasionally on Days 3-5. The addition of new mares to the herd did not induce herding of the original mares. However, the new mares maintained mean distances of 8-12 mare lengths from the original mares, resulting primarily from chasing by the stallion. By Day 4, the distances between the new and original mares were not different (P > 0.05) from the distances among the original mares.     

Use of an androgenized mare as an aid in detection of estrus in mares

A study was conducted over a 2-mo period to compare estrus detection results obtained using an androgenized teaser mare with those obtained with a stallion, using the same group of 10 normally cyclic mares. The teaser mare was androgenized by administration of boldenone undecylenate (500 mg i.m. every 1 to 2 wk), and allowed to run loose with the mare group. Estrus was determined by observation of the group for a 30-min period daily. In the second month of the experiment, a marking harness was used on the androgenized mare to help detect mares mounted when in estrus. Estrous periods detected by each teasing method were 1) first month: stallion, 18; androgenized mare, 5; 2) second month: stallion, 16; androgenized mare, 9. There were no estrous periods detected by the androgenized mare that were not also detected by the stallion. Under these conditions, the androgenized mare was not an adequate estrus detection aid. Also discussed are the successful results of an independent trial on a breeding farm using an androgenized mare as an estrus detection aid.     

Female–female competition or male mate choice? Patterns of courtship and breeding behavior among feral horses (Equus caballus) on Assateague Island

Previous research on the Assateague horses found that high-ranking females had more surviving offspring than low-ranking females. Variance in reproductive success may be the result of a variety of proximate processes that affect sexual behavior such as mate choice and mate competition. A study was done to determine whether patterns of courtship, social, and sexual behavior could be identified that would suggest mate choice and/or mate competition. Behavioral data were collected from approximately 40 sexually mature mares living in harem bands. Stallions showed more interest in the eliminations of dominant mares than subordinate mares. Males also engaged in significantly more high-intensity (e.g., mounts and copulations) sexual behavior with dominant mares than subordinate mares, and there was a trend for males to engage in more low-intensity (e.g., flehmen and ano-genital sniffing) sexual behavior with dominant mares than subordinate mares. There was no effect of mare rank on spatial relationships with the stallion; however, dominant mares did attempt to restrict reproductive access to the stallion by harassing and disrupting copulations. Higher foaling rates among dominant mares on Assateague Island could therefore be the result of rank-related mate choice by stallions and direct female competition for mating opportunities.     

Assessment of the sexual behavior of pregnant mares

Pregnant mares (N = 12) were observed with a stallion from early gestation until parturition for sexual behavior. Observations were conducted for 20 min per day for 5 days each month from September until July. No mares exhibited full estrus and no intromissions or ejaculations occurred at any time during the study. Social interactions such as male approach and female/female mutual grooming occurred with greater mean frequency or duration in some months of spring and fall than in winter months. (P less than 0.05). None of the recorded behaviors differed by month of gestation. In a comparison of the behavior of diestrous and pregnant mares in harem groups, no estrous behavior occurred and no differences in social interactions were found. Although progesterone may be sufficient to cause the absence of sexual behavior in diestrous mares, another estrous-inhibiting substance may be present during pregnancy at times of high estrogen and low progesterone.     

Studies on the effect of manual massage of the ovaries on the reproductive activity of the mare

The aim of this study was to determine the influence of hand massage of mares' ovaries on breeding activity and hormonal changes in the winter anestrous period and during the luteal phase of the oestrus cycle. The experiment was conducted on 5 experimental and 5 control mares. In winter, (January) the experimental mares underwent 30-sec daily massage of both ovaries, for 30 d, and in summer (August) from the 6th day of the cycle to the occurrence of estrus. The sexual behavior of all mares was determined each day by individual teasing by a vigorous stallion, and the ovaries were checked by palpation per rectum and with an USG. Every second day blood samples were drawn from each mare to determine progesterone and estradiol in the plasma. Ovarian massage during deep winter anestrus had no significant effect on acceleration of the mares' active breeding season. Nevertheless, a higher concentration of estradiol was observed in the experimental group. These differences occurring on the 11th, 17th and 20th days were found to be significant (P ≤ 0.05). It was shown that during the summer period, in the luteal phase of the cycle, ovarian massage shortened the length of the estrous cycle, and ovulation was brought on somewhat earlier.     

Coumestrol and its metabolite in mares' plasma after ingestion of phytoestrogen-rich plants: Potent endocrine disruptors inducing infertility

Phytoestrogens exist in plants that are present in forages fed to horses. They may compete with 17-β estradiol and influence the estrous cycle. Therefore, the objective was to determine whether coumestrol from clover-mixed pastures is present in mare's plasma after their ingestion (experiment I), and when this phytoestrogen was present in mare's plasma after ingestion (experiment II). The effect of a long-term ingestion of phytoestrogens on estrous cycle disruption was assessed (experiment III; clinical case). Experiment I was carried out in nonpregnant anestrous and cyclic Lusitano mares (n = 14) kept on clover and grass-mixed pastures, and supplemented with concentrate and hay or cereal straw. Blood and feedstuff were obtained from November to March. In experiment II, stabled cyclic Lusitano mares (n = 6) were fed for 14 days with increasing amounts of alfalfa pellets (250 g to 1 kg/day). Sequential blood samples were obtained for 8 hours after feed intake on Day 0 (control) and on Days 13 and 14 (1 kg/day alfalfa pellets). Experiment III mares were fed with a mixture of alfalfa and clover haylage for 5 months (group 1; n = 4) or for 9 months (group 2; n = 12). Estrous cycle was determined on the basis of plasma estradiol (E₂), progesterone (P₄), and ultrasound (experiment III). Concentrations of phytoestrogen coumestrol and its metabolite methoxycoumestrol were determined by high-performance liquid chromatography coupled with mass spectrometry. Phytoestrogens decreased in pasture from November until March (P < 0.01) (experiment I), but were always detected in mares' plasma. In experiment II, plasma-conjugated forms of coumestrol and methoxycoumestrol were higher on Days 13 and 14 than in control (P < 0.05). The highest concentrations of conjugated form of coumestrol were at 1.5 and 4 hours (P < 0.001), whereas its free forms peaked at 1 and at 3.5 hours after ingestion (P < 0.05). Methoxycoumestrol-conjugated form concentration was the highest at 1.5 and 5 hours (P < 0.001), whereas its free form peaked at 1 hour (P < 0.05) and at 1.5 hours (P < 0.001). Long-term intake of coumestrol caused lack of ovulation, uterine edema, and uterine fluid accumulation (experiment III). Coumestrol and methoxycoumestrol in both forms were higher in group 2 (while still ingesting haylage) than in group 1, after haylage withdrawal (P < 0.001). These data show that in the mare, coumestrol and its metabolite increase in blood after ingestion of estrogenic plants and can influence reproduction in mares as potent endocrine disruptors.     

Novel observation of play behaviour between a harem holder and a bachelor group of Przewalski’s horses in the wild

acta ethologica - Przewalski’s horses live in stable nonterritorial families (harem) of one or more harem stallions, several mares, and their offspring. The harem stallion typically behaves...     

Adrenal contribution to the induction of sexual behavior in the female musk shrew.

Sexually experienced female musk shrews (Suncus murinus) lack an ovarian, vaginal, and behavioral estrous cycle. Females, once induced by their initial contact with a male, are able to display copulatory behavior whenever a male is present (Rissman, Silveira, and Bronson, 1988). Based on plasma levels of steroids, and on hormone replacement studies conducted after ovariectomy (OVX), we have shown that testosterone (T) plays an essential role in the regulation of female sexual behavior (Rissman and Crews, 1988; Rissman, Clendenon, and Krohmer, 1990a; Rissman, 1991). To date we have only examined the potential contribution of adrenal steroids to female sexual behavior in a preliminary manner. After adrenalectomy, gonadally intact females display significantly lower levels of sexual behavior than controls (Rissman and Bronson, 1987). The following experiments were conducted to examine the role the adrenal steroids (in contrast to the medullary hormones) play in the induction of female sexual behavior in the musk shrew. In the first experiment gonadally intact females were treated with dexamethasone (DEX) to reduce the secretion of adrenal steroids. Significantly fewer females receiving DEX demonstrated sexual behavior as compared with controls. In the second study, OVX females received T-filled Silastic implants. When DEX was administered to OVX + T females at a dose that dropped circulating T levels to those found in ovary and adrenal intact females, no effect on sexual behavior was noted. The data show that the adrenals are a behaviorally important source of T and contribute toward the hormonal control of sexual behavior in these female mammals.     

Effects of inbreeding and other systematic effects on fertility of Black Forest Draught horses in Germany

Background

The Black Forest Draught horse (BFDH) is an endangered German coldblood breed with its origin in the area of the Black Forest in South Germany. In this retrospective study, the influence of the inbreeding coefficient on foaling rates was investigated using records from ten breeding seasons. Due to the small population size of BFDH, the level of inbreeding is increasing and may have an effect on foaling rates.The data of the present study included all coverings reported for 1024 BFDH mares in the years 2001–2009. These mares were covered by 32 BFDH stallions from the State Stud Marbach. Data from 4534 estrus cycles was used to calculate per cycle foaling rate (CFR). Pedigree data contained all studbook data up to the foundation of the breed as early as 1836. The level of inbreeding of the mare, stallion and expected foal along with other systematic effects on CFR were analysed using a generalized linear mixed model approach. Stallion was employed as a random effect. Systematic fixed effects were month of mating, mating type, age of the mare and stallion, reproductive status of the mare and stallion line of the mare. Inbreeding coefficients of the stallion, mare and expected foal were modelled as linear covariates.

Results

The average CFR was 40.9%. The mean inbreeding coefficients of the mares, stallions and expected foals were 7.46, 7.70 and 9.66%. Mating type, age of the mare, reproductive status of the mare and stallion line of the mare had a significant effect.

Conclusions

The results showed that the mating type, stallion line of the mare, sire, age and reproductive status of the mare exerted the largest influences on CFR in BFDH. Inbreeding coefficients of the stallion, mare and expected foal were not significantly related with CFR.

     

Experimental simulation of an estrous cycle — gonadotropin surges in estradiol‐infused ovariectomized rats

Abstract

Dynamic relations between the circulating estrogen and the hypophyseal gonadotropin secretion in the estrous cycle were investigated by replacing the ovaries by an infusion pump in freely moving rats. Female rats were ovariectomized in the morning at certain stages of the 4‐day estrous cycle, and simultaneously infused with estradiol (E₂) at a constant rate of 0.35 ng/min up to 120 h through a cannula chronically inserted into the jugular vein. They were killed at 6 h‐intervals. Rats ovariectomized at the second day of diestrus and at estrus showed a sharp rise in LH 36 h and 84 h, respectively, after the initiation of E₂ infusion, when the proestrous surge would occur in normal rats. During the other periods, blood levels of LH were very low, exhibiting a small daily rise in the evening. Similarly ovariectomized rats infused with vehicle only showed a gradual rise of gonadotropin secretion, never reaching the surge level. Rats ovariectomized at proestrus and infused with E₂ showed a LH surge 12 h later as expected. However, surge‐like LH secretions followed every evening thereafter. Thus, the constant supply of E₂ alone could simulate at least one 4‐day cyclic LH surge in ovariectomized rats. E₂ infusion caused a daily peak of FSH synchronized with the LH rises, but could not suppress the post‐operative hypersecretion. It is discussed that if the suppressing effect of progesterone endogenously secreted from the ovaries is cleared, a circadian pattern of the LH/FSH surge may appear under the signal from the cerebral clock mechanism and the effect of circulating estrogen. The failure to suppress the FSH hypersecretion by E₂ might indicate the involvement of inhibin in the regulatory mechanism. Time‐course changes in uterine and vaginal weights are also dealt with and discussed in relation to the constant E₂ exposure.