Ovulation response to pregnant mares' serum gonadotrophin in prepubertal ewe lambs of different Booroola genotypes

Ovulation rates following treatment with PMSG were measured in prepubertal ewe lambs of known Booroola genotype to evaluate the technique as a method of early identification of Booroola genotypes. Two experiments were conducted with homozygous (FF), heterozygous (F+) and non-carrier (++) interbred $\text{1}\text{2}$ Merino $\text{1}\text{2}$ Romney breed type ewe lambs and a third experiment compared first cross F+ $\text{1}\text{2}$ Merino $\text{1}\text{2}$ Coopworth breed type with contemporary Coopworths (++). The ewe lambs aged 5–6 months were injected with 400 or 600 i.u. PMSG. The percentage of ewe lambs ovulating following treatment with PMSG was similar in all experiments (75–77%) but in Experiment 1 the FF and F+ lambs had a significantly higher proportion ovulating than the ++ lambs (P<0.05). The mean ovulation rate of FF ewe lambs treated with PMSG was 1.03 (Expt 1) and 1.69 (Expt 2) higher than ++ lambs (P<0.05). In Experiment 1 the mean ovulation rate of F+ ewe lambs was 0.33 higher than ++ ewe lambs but this difference was not significant. However, in Experiments 2 and 3 the mean ovulation rate of F+ ewe lambs treated with 600 i.u. PMSG was 1.06 and 0.45 higher than ++ ewe lambs respectively (P< 0.05). These results show that PMSG treatment is a promising technique for identifying the Booroola genotype of ewe lambs at an early age but further experiments with different dose rates of PMSG and different ages and liveweights of lambs are required to determine optimum dose rates and time of treatment.     

Pituitary response to GnRH and ovariectomy in Booroola-Awassi and Awassi ewe lambs

Booroola-Awassi ewe lambs were heterozygous (F+) for a major gene F, influencing their ovulation rate, while Awassi lambs were non-carriers (++). Basal plasma FSH concentration (mean +/- s.e.m.) in Booroola-Awassi ewe lambs at 4 weeks of age was significantly higher than in Awassi lambs of the same age (5.06 +/- 0.60 and 2.04 +/- 0.32 ng/ml respectively; P less than 0.001). After GnRH administration, FSH increased from 3.89 +/- 1.10 to 10.58 +/- 1.30 ng/ml in Booroola-Awassi (N = 6) and from 1.87 +/- 0.29 to 4.64 +/- 0.33 ng/ml in Awassi (N = 6) ewe lambs (P less than 0.05). Ovariectomy caused an increase in plasma FSH in Booroola-Awassi (N = 4) and Awassi (N = 4) ewe lambs. At 1 week after ovariectomy plasma FSH increased from 5.96 +/- 1.02 to 7.06 +/- 1.05 ng/ml in F+ and from 1.67 +/- 1.06 to 5.21 +/- 0.66 ng/ml in ++ ewe lambs, suggesting a stronger negative feed-back effect exerted by the ovaries of Awassi lambs. At 15 weeks after ovariectomy FSH values were similar in Booroola-Awassi (18.28 +/- 1.96 ng/ml) and Awassi (16.07 +/- 0.70 ng/ml) lambs. Although the overall pattern of pituitary response to ovariectomy was similar in the F+ and ++ ewe lambs, Booroola-Awassi lambs had small ovaries (132.5 +/- 24.9 mg) and follicular development did not proceed beyond the preantral stage in 3/4 animals, and Awassi lambs had large ovaries (600.0 +/- 233.9 mg) (P less than 0.05) with many preantral and antral follicles.(ABSTRACT TRUNCATED AT 250 WORDS)     

Survival of fertilized ova from ewe lambs and adult ewes in the uteri of ewe lambs

The cause of fertility differences between ewe lambs and adult ewes following natural oestrus were studied in Romneys. Insemination was determined by anterior vaginal swabbing. Fertilized ewe-lamb ova were returned to donor animals along with one matched fertilized adult ewe ovum and lambings were recorded. Ewe-lamb ova were less likely (P<0.01) to survive to term compared with adult ewe ova. Insemination failure, fertilization failure and anovular oestrus were minor sources of reproductive wastage.It is concluded that reduced ovum quality appears to be the major cause of the lower fertility in naturally ovulating ewe lambs compared with adult ewes.     

Effect of PMSG dosage on the reproductive performance of adult ewes and ewe lambs bred at a progestagen-PMSG synchronized estrus

The effect of dose of pregnant mares' serum gonadotropin (PMSG) on the reproductive performance of adult ewes and ewe lambs and lamb survival at birth after treatment with fluorogestone acetate (FGA)-impregnated intravaginal sponges and PMSG (250 IU or 500 IU) to synchronize estrus was evaluated. Ewes were exposed to rams for breeding at the synchronized and subsequent estrous cycles. The flock, comprised of three synthetic strains and two control breeds, was maintained in a controlled environment and exposed to an artificial light regimen which alternated at 4-mo intervals from 16h of light daily to 9h of light daily. Trials were conducted during January, May and September at the end of a 9-h daylength cycle. Adult ewes were bred in May and 8 mo later in January. Ewe lambs were bred in September at 6.5 to 7.5 mo of age. The overall reproductive performance of the adult ewes was similar at the two breedings: fertility approximately 90%, prolificacy approximately 2.7, fecundity approximately 240% and lambs born alive approximately 2.4. Dosage of PMSG had no effect. Reproductive performance of ewe lambs was lower and there was a strain x treatment interaction, suggesting greater variability in response. The results indicate there is no advantage to using a higher dose of PMSG in ewes with a natural relatively high fecundity. Moreover, the use of the artificial photoperiod appears to overcome the natural seasonal variation in reproductive performance.     

Pineal and ovarian response to 22- and 24-h days in the ewe

Melatonin secretion in ewes was entrained by 22-h light-dark cycles whether of long (16L:6D) or short (6L:16D) photoperiod. In photoperiods of 6L:16D, a phase-delay of melatonin secretion was evident, leading to a dark-phase duration shorter than that found in 8L:16D. Early onset of estrus was induced in anestrous ewes kept in 8L:16D, but not 6L:16D, from 22 July compared to controls in natural light. In photoperiods of 16L:6D, the melatonin profile corresponded precisely to the dark phase. Early offset of estrus was induced in estrous ewes kept in both 18L:6D and 16L:6D from 18 December compared to controls in natural light. Thus, when the duration of melatonin secretion was appropriate to the long photoperiod (16L:6D), but with a constantly changing phase position, a long-day reproductive response was found. Activity-rest cycles were not entrained by 16L:6D; thus the synchronization of melatonin and activity-rest cycles does not appear to be essential for the induction of a long-day reproductive response. These results support the hypothesis that the duration, not the circadian-phase position, of melatonin is critical to the induction of photoperiodic effects.     

Oestrous and ovarian responses to exogenous progesterone and oestrogen in prepubertal ewe lambs in relation to the fecundity of their dams

Oestrus was consistently induced by 20, 40 or 80 microgram oestradiol benzoate in progesterone-primed lambs aged 23 weeks and 17 weeks respectively in two experiments. The duration of oestrus increased linearly with increasing log dose of oestrogen, and was negatively correlated with the time of its onset. In the absence of progesterone there was a reduced incidence, later onset and shorter duration of oestrus. Progesterone alone did not induce oestrus. In lambs treated during January (Exp. 2), a later date of injection of oestrogen was associated with earlier onset and longer duration of oestrus. The induced oestrus was anovulatory. Oestrogen reduced the proportion of lambs with follicles greater than or equal to 3 mm in diameter whilst progesterone had no effect. Lambs which were the progeny of low- and high-fecundity dams did not differ in their oestrous or ovarian responses. Correlations between the dam's lamb production index and the time to onset and duration of the induced oestrus were also not significant.     

Growth response, endocrine profiles and reproductive performance of fine-wool ewe lambs treated with ovine prolactin before breeding

Twenty-four 6-mo-old ewe lambs received one of two ovine prolactin (oPRL) treatments 28 d before fall breeding. Beginning on the first day of treatment (Day 0), 12 lambs received a subcutaneous injection (12 ml) of a carrier vehicle (0 mg oPRL) on alternate days for 28 d while 12 lambs received injections containing 5 mg oPRL. On Days 0 and 28, jugular blood was collected from six lambs in each group before treatment and at 30-min intervals for 6 h thereafter. Neither feed intake, efficiency of gain nor animal weights differed (P > 0.20) between groups. One hour after treatment on Day 0, ewe lambs receiving 5 mg oPRL had greater (P < 0.10) serum PRL levels than did controls (121.9 and 61.5 +/- 24.7 ng/ml, respectively). Differences in serum PRL persisted throughout remaining sampling intervals on both Days 0 and 28. Serum samples obtained on alternate days during the 28-d treatment period revealed no differences (P > 0.20) in PRL concentrations between control (48.3 +/- 5.3 ng/ml) and oPRL-treated (55.7 +/- 5.3 ng/ml) ewes. Neither serum insulin nor growth hormone responded (P > 0.05) to exogenous oPRL on either Day 0 or 28. No difference (P > 0.30) in percentage of ewe lambs cycling during treatment or breeding was detected between groups. Subsequent lambing percentages were similar (P > 0.30), with 36.4% of control and 25.0% of oPRL-treated ewes producing offspring. Administering 5 mg oPRL on alternate days for 28 d before breeding did not enhance growth and(or) reproductive performance in virgin ewe lambs.     

Pituitary response to LHRH, LH pulsatility and plasma melatonin and prolactin changes in ewe lambs treated with melatonin implants to delay puberty

Prepubertal ewe lambs were treated with empty or filled melatonin implants. The implants were placed s.c. at birth and pituitary responsiveness to various doses of LHRH, LH/FSH pulsatility and prolactin and melatonin secretion were examined at 10, 19, 28, 36 and 45 weeks of age. Control animals (N = 10) showed no consistent alteration in pituitary responsiveness to LHRH during development. Ewes treated with melatonin (N = 10) had puberty onset delayed by 4 weeks (P less than 0.03) but no effect of melatonin on LH or FSH response to LHRH injection was observed at any stage of development. In the control and melatonin-treated ewe lambs the responses to LHRH injection were lower during darkness than during the day at all stages of development. No consistent differences in LH or FSH pulsatility were observed between treatment groups or during development. Prolactin concentrations, however, failed to decrease at the time of puberty (autumn) in the melatonin-treated group. Melatonin-treated ewe lambs maintained normal rhythmic melatonin production which was superimposed on a higher basal concentration and showed the same increase in melatonin output with age as the control ewes. These results indicate that the delayed puberty caused by melatonin implants is not due to decreased pituitary responsiveness to LHRH or to dramatic changes in basal LH or FSH secretion.     

Luteinizing hormone response to estradiol benzoate in cows postpartum and cows with ovarian cysts

Twenty-seven dairy cows were evenly assigned to one of three groups and given an intramuscular injection of 2 mg estradiol benzoate. Cows in group 1 were greater than 30 days postpartum at treatment and had been diagnosed via rectal palpation to have ovarian cysts. Cows in groups 2 and 3 were 12 to 14 and 30 to 40 days postpartum, respectively. Blood plasma was collected from all cows before treatment and then every three hours for 36 hours post-treatment. Concentrations of LH, estradiol-17 beta and progesterone in plasma were determined by radioimmunoassay. Four, zero and five cows in groups 1, 2 and 3, respectively, had concentrations of progesterone greater than 1.0 ng/ml before estradiol benzoate treatment. None of these cows had a peak LH release greater than 5 ng/ml following estradiol benzoate treatment. The numbers of cows with progesterone concentrations less than 1 ng/ml that released LH (>5 ng/ml) in response to estradiol benzoate were 3 of 5, 3 of 9, and 4 of 4 for groups 1, 2, and 3, respectively; the proportion for group 3 was higher (P<.05) than for group 2. Of the cows that released LH, mean peak LH concentrations were 33.3+/-5.4, 14.8+/-7.2 and 24.6+/-9.8 ng/ml for groups 1, 2 and 3, respectively, and the duration of the LH increase was 8.0+/-1.0, 8.0+/-2.0 and 13.0+/-4.0 hours. The time from estradiol benzoate treatment to peak LH release for cows with ovarian cysts (25+/-2 hours) was delayed (P<.05) compared with that for cows 30 to 40 days postpartum without ovarian cysts (16+/-1 hour). In summary, responsiveness to estradiol benzoate is regained between 2 to 4 weeks postpartum in most cows. In addition, some cows with ovarian cysts can release LH in response to estradiol benzoate, but peak LH release is delayed compared to cows at a comparable stage postpartum without ovarian cysts.     

Effects of rapid weight gain to puberty on reproduction, mammary development and lactation in ewe lambs

The effects of rapid weight gain to puberty on reproduction, mammary development and milk production in ewes lambing at 13 mo of age were investigated on three trials. A total of 64 Dorset and 93 Suffolk ewe lambs were weaned at 42 d of age and their mean weight was 16 kg. These ewes were assigned, within breed groups, to either a finishing diet or a growing diet. Onset of puberty was determined by daily checks for estrus and ewes were bred beginning at 7 mo of age. In Trial 2, mammary gland development was determined in eight Suffolk ewes from each diet. Ewes on the finishing diet were younger at puberty than those on the growing diet (199 vs 206 d, P<0.05) but required more services per conception (1.3 vs 1.1, P<0.05). Dietary conception rate and lambing rate means were similar. Mean 4-h milk yield was lower (P<0.10) for ewes on the finishing diet (283 g) than for those on the growing diet (310 g). Mammary gland fat pad area was higher (P<0.05) for ewes on the finishing diet compared with those fed for growth. Gross and adjusted duct areas were higher in ewes on the growing diet, but differences were not significant. At puberty, negative correlation coefficients for milk yield with performance traits were as follows: daily weight gain, -0.184 (P<0.08); weight-to-height ratio -0.262 (P<0.01); body condition score, -0.189 (P<0.07); and body weight, -0.212 (P<0.05). Results of this study indicate that rapid weight gain to puberty impairs mammary gland development and milk production in ewe lambs.     

The effect of hCG treatment on Day 12 post-mating on ovarian function and reproductive performance of ewes and ewe lambs

The objectives of this study were to determine the effect of a single intramuscular injection of 200 IU hCG (Chorulon) on Day 12 post-mating on ovarian function and subsequent lambing performance in ewes and ewe lambs bred at synchronised oestrus during the breeding season and on the lambing performance of ewes induced to breed during late anoestrus. All animals were mated to rams at synchronised oestrus and on Day 12 post-mating given normal saline or 200 IU hCG.In Experiment 1, laparoscopic results showed that hCG treatment induced accessory corpora lutea in ewes (control = 0/7; hCG = 5/7) but not in ewe lambs (control = 0/7; hCG = 0/7).In Experiment 2, hCG treatment did not improve the lambing rate (control = 50; hCG = 57) or the litter size (control = 1.80; hCG = 1.96) in ewes (control = 100; hCG = 91). However, hCG treatment significantly (P > 0.05) improved the lambing rate (control = 29; hCG = 58; P < 0.05) in ewes conceiving at the first oestrus after treatment. hCG treatment (control = 42; hCG = 42) also failed to improve the lambing rate in ewe lambs (control = 48; hCG = 41).In Experiment 3, hCG treatment had no significant (P > 0.05) effect on the lambing rate (control = 72; hCG = 62) or the litter size (control = 1.59; hCG = 1.58) in ewes (control = 111; hCG = 115) induced to breed during anoestrus or on ewes returning to oestrus and conceiving after treatment (lambing rate: control = 86; hCG = 72; litter size: control = 1.44; hCG = 1.35). In conclusion, the data obtained in this study suggest that during the breeding season hCG may, by stimulating ovarian function, improve embryo survival in ewes conceiving at the first post-treatment oestrus. This effect, however was not observed in ewe lambs.     

Influence of dose, repeated treatment and batch of hormone on ovarian response in heifers treated with PMSG.

The effect of two dose levels (1000 and 2000 i.u.) of three different commercially available batches of PMSG on the ovarian response (ovulations and follicles greater than 10 mn) of 42 heifers was examined in a randomized incomplete block experiment. Each animal was subjected to two consecutive but different treatments. A significant effect of dose was observed and there were fewer ovulations, but no reduction in the number of follicles, after the second PMSG treatment. There was no evidence that the ovarian response was affected by the PMSG batch used.     

Testosterone and estradiol potentiate the serum gonadotropin response to gonadotropin-releasing hormone in goldfish

The effects of gonadal steroids on gonadosomatic index (GSI; gonad wt/total body wt x 100), pituitary gonadotropin (GTH) content, and serum GTH response to [D-Ala6,Pro9-Net]-luteinizing hormone-releasing hormone (LHRH-A) were investigated throughout the seasonal reproductive cycle of the goldfish. Gonad-intact female fish were implanted i.p. for 5 days with silastic pellets containing no steroid (blank), testosterone (T; 100 micrograms/g), or estradiol (E2; 100 micrograms/g). The serum GTH response at 6 h following i.p. injection of saline or 0.1 microgram/g LHRH-A was assessed. In blank-implanted, saline-injected animals, seasonal variations in GSI, pituitary GTH content, and serum GTH levels were evident; maximal and minimal levels were noted in the spring and summer months, respectively. In blank-implanted fish, LHRH-A effectively stimulated GTH release in females undergoing gonadal recrudescence (late autumn and winter) and in sexually mature (spring) females, but not in sexually regressed (summer and early autumn) females. Implantation of T or E2 raised serum steroid levels to those found during ovulation in goldfish. Steroid treatments did not affect unstimulated serum GTH levels at any time of the year. Testosterone effectively potentiated the serum GTH response to LHRH-A during the entire reproductive cycle, whereas the positive effects of E2 were evident in sexually regressed and post-spawning females only. Both T and E2 potentiated the GTH response to LHRH-A in male fish. To examine the involvement of T aromatization in mediating its actions on induced GTH secretion, male and female fish were implanted with T or the nonaromatizable androgens 5 alpha-dihydroxytestosterone (DHT; 100 micrograms/g) and 11-keto-testosterone (11-KT; 250 micrograms/animal). Testosterone potentiated the GTH response to LHRH-A in both males and females whereas DHT and 11-KT were without effect. Furthermore, the positive action of T on induced GTH secretion was blocked by 2-day pretreatment with the aromatase inhibitor 1,4,6-androstatrien-3,17-dione (100 or 300 micrograms/g). Multiple i.p. injections of hCG (0.2 microgram/g every 3 days for 39 days), probably through stimulation of endogenous T secretion, resulted in potentiation of the GTH response to LHRH-A in mature male goldfish. These results clearly demonstrate that T, through aromatization to E2, can increase pituitary responsiveness to exogenous LHRH-A in gonad-intact male and female goldfish.     

Effect of maternal exposure to the environmental estrogen,octylphenol, during fetal and/or postnatal life on onset of puberty,endocrine status,and ovarian follicular dynamics in ewe lambs

Octylphenol (OP) is one of a number of compounds found in the environment that has estrogen-mimicking actions in vivo. Our objective was to determine if maternal exposure to octylphenol during fetal and/or postnatal life would affect the onset of puberty, endocrine status, and subsequent ovarian follicular dynamics of ewe lambs. Lambs were born in March to ewes that received twice weekly s.c. injections of octylphenol (1000 micro g/kg/day) from Day 70 of gestation to weaning (n = 6); Day 70 of gestation to birth (n = 3); birth to weaning (n = 5; gestation = 145 days); or corn oil from Day 70 of gestation to weaning (control; n = 5). Blood samples were collected twice weekly to determine progesterone and FSH concentrations from 20 wk of age throughout the first breeding season. Onset of puberty and interestrous intervals were determined from 20 wk of age by twice daily observation for estrus in the presence of a vasectomized ram. During January the ovaries of each lamb were examined using transrectal ultrasonography from the day of estrus for 15 days. Blood samples were collected every 8 h to examine FSH concentrations and every 2 h to detect the preovulatory gonadotropin surge throughout this estrous cycle. The onset of puberty and first progesterone rise was advanced and the FSH preovulatory surge was elevated for longer in the OP-treated lambs compared with the control lambs (P < 0.05). Interestrous intervals, FSH profiles, and ovarian follicular dynamics were not affected (P > 0.05) by exposure to octylphenol. In conclusion, octylphenol exposure advanced the onset of puberty but it did not disrupt FSH concentrations or the dynamics of ovarian follicular growth.     

Age at puberty, fertility and litter size of ewe lambs reared under different photoregimens

Age at puberty, fertility and litter size of ewe lambs of synthetic sire and dam strains raised under different photoregimens were determined. The lambs were bred during January, May or September at 30 to 32 weeks of age. Irrespective of birth date, the lambs were reared under continuous light from birth to 5 weeks of age. From 5 to 20 weeks of age, they were kept under 16 hours of light dairy (16L:8D; Treatment A), 8 hours of light daily (8L:16D; Treatment B), or a split photoperiod of 8 hours total light daily (7L:9D:1L:7D; Treatment C). Subsequently, all lambs were exposed to 9 hours of light daily until after breeding. Lambs were exposed to rams for two estrous periods after treatment with fluorogestone acetate-impregnated intravaginal sponges and pregnant mares' serum gonadotropin (PMSG) to induce synchronized estrus. Although the age at puberty (174 days) was similar among treatments, the incidence of puberty prior to progestagen sponge treatment was higher ( approximately 50%) for lambs reared under Treatments A and C than under Treatment B. Fertility and litter size of lambs were not influenced by the previous photoperiod history or by sexual maturity, i.e., puberal or prepuberal, at the start of the sponge treatment. However, strain, age and weight of lambs at breeding influenced significantly the reproductive outcome.     

Ability of ram introduction to induce LH secretion, estrus and ovulation in fall-born ewe lambs during anestrus.

The ability of ram introduction (RI) and progesterone pre-treatment to induce increases in LH secretion and ovulation, and the ability of progesterone pre-treatment with or without estrogen to induce estrus and ovulation in fall-born ewe lambs during seasonal anestrus was investigated. In early July, lambs of mixed breeds (41.8+/-0.6 kg and 250.7+/-1.3 days of age) were assigned to receive no treatment (C, n=7), to be introduced to rams (7:1 ewe:ram ratio; R, n=7), to be treated with progesterone (a used CIDR device) for 5 days (P, n=5), to be treated with progesterone and introduced to rams at CIDR removal (PR, n=11), or to receive the latter treatment plus an injection of estradiol benzoate (25 microg, E2beta i.m.) 24 h after CIDR withdrawal/RI (PER, n=11). Blood samples were collected from all lambs every 4h for 60 h beginning at RI/CIDR withdrawal (0 h), to characterize the LH surge profile and in groups R and C every 15 min for 8 h between 12 and 20 h for determination of LH pulse frequencies. Ultrasonographic examinations of the ovaries were conducted at 0, 36 and 60 h. In ram-exposed groups lambs were also observed for raddle marks every 4h from 0 to 60 h. The LH pulse frequency (pulses/8 h) was higher in group R (P<0.01; 7.7+/- 0.5) than group C lambs (2.7+/- 0.8). More lambs in groups exposed to rams than in the C or P groups showed an LH surge (P<0.05; 0, 100, 0, 72.7 and 100%, for C, R, P, PR and PER groups, respectively). Time from RI/CIDR removal to initiation of the LH surge was greater in lambs in the PR (43.5+/- 3.8h) than in the R (32.6+/- 4.6h; P=0.08) or PER (33+/- 1.2h; P<0.01). Diameter of the largest follicle at 0 h (3.2+/- 0.2mm) was not different among groups. Growth rate of the largest follicle between 0 and 36 h was greater (P<0.05) in RI than in C or P groups. Diameter of the largest follicle at 36 h was larger (P<0.05) in lambs in R (5.6+/- 0.2mm) and PR (5.1+/- 0.5mm) than C (4.0+/- 0.6mm) or P (3.8+/- 0.4mm) groups, and in R than PER (4.3+/- 0.4mm) treatment groups. Only lambs in the RI groups ovulated. Among RI groups the percentage of lambs ovulating was greater in the R (P<0.05; 85.7%) than PR (33.3%) groups with an intermediate response observed in lambs in treatment group PER (71.4%). The estrous response in progesterone pre-treated groups was greater (P<0.05) in lambs also treated with estrogen (PER; 81.8%), than in lambs introduced to rams alone (PR; 45.5%). In conclusion, ram introduction by itself, but not progesterone treatment alone, induces increases in LH pulse frequency, follicular development, and ovulation in fall-born ewe lambs during seasonal anestrus, further, P4 pre-treatment and RI when combined with E2 results in a high estrous response.     

Progesterone blocks the estradiol-stimulated luteinizing hormone surge by disrupting activation in response to a stimulatory estradiol signal in the ewe

The preovulatory surges of GnRH and LH are activated by increased concentrations of circulating estradiol, but ovulation is blocked when progesterone concentrations are elevated. Although it is has been shown that this action of progesterone is due to a central inhibition of the GnRH surge, the mechanisms that underlie the blockade of the GnRH surge are poorly understood. In this study we investigated whether progesterone can block the estradiol-dependent activation stage of the GnRH surge induction process, and thus prevent expression of the LH surge. The results demonstrated that exposure to progesterone for half or the full duration of the activation stage can prevent the stimulation of LH surges by estradiol (experiment 1), whereas exposure to progesterone midway though a period of estradiol exposure, which in itself is sufficient to activate the surge, did not block the LH surge (experiment 2). These results suggest that progesterone 1) disrupts activation of the surge induction system in response to a stimulatory estradiol signal and 2) does not compromise the ability of animals to respond to a stimulatory estradiol signal applied immediately after progesterone exposure. Because the disruptive effects of activated progesterone in response to estradiol are rapid but transient, it may be that progesterone directly interferes with the activation of estradiol-responsive neural systems to block the GnRH/LH surge.