Ovarian response to PMSG treatment in ewes immunized against oestradiol-17 beta

The effects of active immunization against oestradiol-17 beta on the ovarian response to pregnant mare serum gonadotrophin (PMSG) was investigated in Merino ewes. Immunized (79) and control (41) ewes were synchronized with intravaginal sponges, given either 750 or 1500 i.u. PMSG and then mated to rams or inseminated laparoscopically with fresh diluted semen. All control ewes mated naturally exhibited oestrus and 40 out of 41 control ewes ovulated. The ovulation rate was higher in the controls receiving 1500 i.u. PMSG than in those ewes which received 750 i.u. PMSG (10.2 v. 3.3). Immunization against oestradiol-17 beta resulted in antibody titres varying from 100 to more than 100 000 in plasma taken 1-4 days after mating. The ovarian response increased significantly in the lowest titre group (100-1000) in conjunction with stimulation with 1500 i.u. PMSG. In these ewes the ovulation rate increased over controls (16.7 v. 10.2) as did the total ovarian response, which includes follicles greater than 10 mm diameter (22.3 v. 11.1). The total ovarian response was also increased in those ewes given 750 i.u. PMSG which had titres in the 1000-10 000 and 10 000-100 000 range, but this was not accompanied by significant increases in the ovulation rate. In general, the higher titre levels (greater than 1000) were correlated with decreases in the proportion of ewes showing oestrus and ovulating and in the embryo recovery rate. The 1500 i.u. PMSG treatment group with the highest titres (greater than 10 000) also showed a significant drop in the ovulation rate as compared to the 1500 i.u. PMSG controls.     

Effects of PMSG on progesterone levels in ewes treated with fluorogestone acetate or prostaglandin F2α

Twenty-four crossbred ewes were observed for oestrus during January. Blood samples, taken on days 0, 1, 2, 5, 9, 13, 15 and 17 of the oestrous cycle, were assayed for progesterone to establish a pretreatment (control) progesterone profile. In the middle of February, 12 ewes were treated with 40 mg fluorogestone acetate (FGA) impregnated intravaginal sponges for 12 days. At the time of sponge withdrawal, six ewes were treated with an intramuscular (IM) injection of 500 i.u. pregnant mares' serum gonadotrophin (PMSG). Twelve other ewes were treated with two injections of prostaglandin F_2α (PGF_2α) 15 mg each, 11 days apart; six ewes were treated with 500 i.u. PMSG IM at the time of the second PGF_2α injection. All ewes were observed for oestrus and blood samples, taken on days 0, 1, 2, 5, 9, 13, 15 and 17 of the first post-treatment oestrous cycle, were assayed for progesterone to establish a post-treatment progesterone profile. PMSG significantly increased the total progesterone in ewes treated with FGA but not in ewes treated with PGF_2α.     

Controlled sheep breeding: update 1980-1985

This contribution to the Symposium concerns four topics which have been addressed in our laboratory over the past five years. First, the responses to a controlled light environment of Merino ewes and rams have been compared with those of two British breeds. The endocrinological patterns were similar in all breeds but cyclic ovarian activity and ram libido were different. While showing a degree of entrainment to photoperiod, the breeding patterns were much less rigidly controlled in the Merinos than in the others. Second, the effectiveness of establishment of a cervical reservoir of spermatozoa, in ewes in which oestrus and ovulation have been controlled, has been re-examined. This is highly dependent on the time of insemination relative to that of the release of LH. Maximum numbers are found when ewes are inseminated shortly after the LH peak, i.e. some 6-10 h after the onset of oestrus. Third, the quantitative and temporal endocrinological and behavioural events following standard, progestagen-PMSG treatment have been quantified. Contrary to earlier expressed beliefs, these events are remarkably predictable provided an intensive system of mating or detection of oestrus is used. The onset of oestrus in treated anoestrous crossbred ewes has a normal distribution, with a range of 24 h, centred around a mean of 33 h after withdrawal of a 30 mg Cronolone intravaginal sponge and injection of 500 i.u. PMSG. This period of time is dose-dependent. The LH peak occurs 4.5 +/- 0.7 h later and the times of onset of oestrus and of LH release are highly correlated (r = 0.93). Ovulation is some 24 h later again. Fourth, differences in the response of ewes to different batches of PMSG have been defined. While the three commercial preparations studied regularly induced ovulation in anoestrous ewes at doses of 250 i.u. and above, the quantitative responses varied greatly. One preparation would not induce multiple ovulation, even at high doses. There are differences in steroidogenesis and in pregnancy rates, associated with dose of PMSG and the consequent ovulation rate: the ideal would be for every ewe to shed two or three ova. A higher ovulation rate is acceptable, as early embryonic mortality generally reduces the litter size. This is particularly important in deep anoestrus. However, this does not solve the problem of breeding in early lactation.     

Efficacy of exogenous gonadotrophic hormones to induce estrus in anestrous gilts

The objective of this study was to examine the response of anestrous gilts to injections of pregnant mare's serum gonadotrophin (PMSG) alone or in combination with human chorionic gonadotrophin (hCG). One hundred and eighty gilts which had failed to exhibit estrus by about 33 wk of age were given one of the following treatments: no injection, 500 IU PMSG, 1000 IU PMSG or 400 IU PMSG + 200 IU hCG. A greater number of gilts injected with 1000 IU PMSG exhibited estrus within nine days of treatment than control gilts (21/37 vs 13/41, X(2) = 5.0, P<0.05). In addition, gilts injected with 1000 IU PMSG exhibited oestrus significantly earlier than gilts receiving the other treatments. In comparisons of the proportion of gilts ovulating within 9 d of treatment and the treatment-to-ovulation interval, there were no significant differences between the three exogenous hormone treatments. There was also no significant effect of treatment on farrowing rate or subsequent litter size. The results of our study indicate that treatment of anestrous gilts with 1000 IU PMSG effectively induces ovulation and fertile estrus. Inadequate expression of estrus often accompanied the ovulation induced by the lower dosages of PMSG used with and without hCG in this experiment.     

Induction of ovulation and fertilization in the 90-day-old ewe lamb.

The induction of ovulation and fertilization was studied in 50 sexually immature crossbred ewe lambs which received 500 or 1000 i.u. PMSG with or without pretreatment with progesterone. Pretreatment with progesterone did not significantly affect ovulation, fertilization or ova recovery rates. Also, progesterone pretreatment did not significantly affect weight of the ovaries, corpora lutea, follicular fluid or the reproductive tract. Lambs receiving 1000 i.u. PMSG had a significantly (P<.05) higher ovulation rate (13.2) than lambs receiving 500 i.u. PMSG (3.4). Weights of the ovaries, corpora lutea, follicular fluid and the reproductive tract were significantly (P<.05) higher in ewes receiving 1000 i.u. PMSG. None of the lambs exhibited estrus when checked daily following HCG injection. The low ova recovery rate (16 to 38 percent) was postulated to be due to failure of the fimbria of the influndibulum to surround the enlarged stimulated ovary and pick up released ova.     

Progestagens,PMSG and the “ram effect” after artificial insemination in spring to synchronise non-pregnant ewes

A significant proportion of ewes mated in spring do not lamb because they return to anoestrus before conception. In a flock of 661 artificially inseminated Merino ewes, PMSG, the “ram effect” and progestagens were used to stimulate ovulation and oestrus in non-pregnant ewes. Injection of PMSG increased the proportion of non-pregnant ewes returning to oestrus from 56% to 82% (P < 0.001). Progestagen priming increased the proportion of ewes in the flock that lambed from 34% to 45% (P < 0.01). These two findings suggest ways by which the reproductive performance of ewes mated in spring could be improved.     

Comparison of control of oestrus and ovulation in sheep by an ear implant (SC-21009) or by intravaginal sponge (cronolone or map)

Oestrus and ovulation were observed in 234 Galway ewes in the breeding season in a preliminary evaluation of an implant progestagen treatment. A miniature ear implant (3 mg SC-21009) was used in a comparison with two intravaginal sponges (30 mg Cronolone and 60 mg medroxy progesterone acetate (MAP)). Each progestagen treatment was used in conjunction with pregnant mare serum gonadotrophin (PMSG) (0, 375 i.u. and 750 i.u.). The percentage of ewes showing oestrus after treatment was significantly affected by progestagen (Cronolone, 95%; MAP, 71%; SC-21009, 74%; P < 0.01), but not by PMSG dose level (0.1 < P < 0.2). Oestrus onset was similar among treatments, but heats ended significantly earlier in implant sheep. Significantly (P < 0.01) more ewes ovulated following Cronolone treatment (92%) than following MAP (74%) or SC-21009 (71%), and there was also a significant effect of PMSG (0, 73%; 375 i.u., 86%; 750 i.u., 93%; P < 0.01). PMSG dose level had a highly significant effect on the mean ovulation rate (0, 1.23; 375 i.u., 1.52; 750 i.u., 2.12; P < 0.001).     

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.     

Natural and induced ovulation rate in prolific and non-prolific breeds of sheep in Ireland, Morocco and New Zealand

Ovulation rate, in mixed-age groups of prolific and non-prolific ewe breed types, after administration of a range of doses of PMSG (0, 375, 750 and 1500 i.u.) during the follicular phase of the oestrous cycle, were compared in Ireland, Morocco and New Zealand. The ewes in Ireland and Morocco were from the Finnish Landrace and Galway, and D'Man and Timhadite breeds, respectively. In New Zealand Booroola Merino x Romney ewes which had been previously identified as heterozygous carriers (F+) of the Booroola high fecundity gene and purebred Romneys were used to represent the prolific and non-prolific genotypes respectively; in addition a group of Booroola Merino x Romney non-carriers (++) of the major gene were also included for comparison. Ovulation rate at the oestrus which preceded stimulation with PMSG was also measured in all animals. In all 3 locations the ewes of the prolific genotype had a greater ovulation rate after PMSG stimulation than did the non-prolific controls. However, this association between prolificacy and response to PMSG was removed when ovulation rate after PMSG was transformed by dividing by the ovulation rate observed before PMSG administration. Despite the differences in the genetic basis of their high prolificacy the pattern of response to PMSG over the range of dosages used was similar in Finnish Landrace, D'Man and Booroola Merino x Romney (F+) ewes and all breeds had means of about 10 ovulations in response to 1500 i.u. PMSG. Amongst the non-prolific breeds, the Timhadite was the most responsive to PMSG although it had the lowest natural ovulation rate.(ABSTRACT TRUNCATED AT 250 WORDS)     

Duration of oestrus, ovulation rate, time of ovulation and plasma LH, total oestrogen and progesterone in Galway adult ewes and ewe lambs

The mean duration of oestrus, ovulation rate, duration of the preovulatory LH discharge, time interval between sponge removal and beginning of the LH discharge, total LH discharged, maximum LH value observed and the concentration of progesterone in the peripheral plasma during the luteal phase of the oestrous cycle was similar in Galway adult ewes and 8-month-old ewe lambs after treatment with intravaginal sponges containing 30 mg cronolone for 12 days and injection of 500 i.u. PMSG. The interval between sponge removal and the onset of oestrus was shorter for adults than for ewe lambs; the interval between the onset of oestrus and the beginning of the LH discharge was longer in adults. During the period 12-36 h after sponge removal the mean plasma total oestrogen concentration was significantly higher in lambs than in adults. In a separate study of the time of ovulation in Galway ewe lambs given the same progestagen-PMSG treatment, ovulation did not occur in any lamb before 17 h after the onset of oestrus and the majority ovulated close to the end of oestrus.     

Ovarian response, ova recovery and fertility in merino ewes superovulated either during the luteal phase of their oestrous cycle or after intravaginal progestagen treatment

Five groups of merino ewes were treated with 1000 i.u. of pregnant mare serum gonadotropin (PMSG) as a single injection per ewe. Three of these groups received treatment on days 7,9 and 11 of their oestrous cycle. Oestrus was synchronized with 125 mg of prostaglandin F2(alpha) (PG) given two days after PMSG. Oestrus in the other two groups was synchronized by intravaginal progesterone sponges inserted for 14 days. In one group, the sponges were inserted nine days after oestrus onset. In the other group the stage of the oestrous cycle was unknown. In both these groups, PMSG was given a day prior to sponge removal. No significant differences were recorded for either the mean numbers of corpora lutea, unovulated follicles or ova recovery between the five groups. However, progestagen synchronized ewes yielded significantly more fertilized ova (p < 0.05) than PG synchronized ewes.     

Embryo production and endocrine response in ewes superovulated with PMSG, with or without monoclonal anti-PMSG administered at different times

Mature nonlactating Altamurana ewes (n = 168) were synchronized in the seasonal anestrus period with FGA-impregnated intravaginal pessaries for 12 d. In Experiment 1, 48 ewes were divided into a 3 x 4 factorial design for anti-PMSG monoclonal antibody (AP) bioassay test. Concomitant injections of PMSG (1000, 1500, 2000 IU) and AP (0, 1, 2, 3 microl/IU PMSG) were given, and ovarian response was evaluated by laparoscopy. In Experiment 2, 120 ewes were divided into 8 experimental groups (n = 15 per group). The ewes treated with 1000 or 1500 IU PMSG at -24 h from sponge removal were given AP intravenously at 50 h after pessary withdrawal, 12 or 24 h after the onset of estrus, while the controls did not receive AP. Blood samples were collected from ewes (n = 6) treated with 1500 IU PMSG with or without anti-PMSG. Ovarian response and embryo production were evaluated on Day 7 after sponge removal upon laparotomy. It was found that 1 microl AP was effective in neutralizing 1 IU PMSG. No significant differences in serum concentrations of progesterone were observed among the groups of superovulated ewes. Estradiol-17 beta levels were reduced following AP treatment 12 h after the onset of estrus. At a lower dosage of superovulatory treatment (1000 IU PMSG), AP injected at 12 or 24 h after the onset of estrus significantly lowered large follicles (P < 0.01) and increased the rate of ovulation (P < 0.05). Moreover, embryo production showed a more than two-fold increase (P < 0.01) of viable embryos following AP injection at 12 or 24 h after the onset of estrus (3.2 to 3.3 vs 1.3, with vs without anti-PMSG). It is concluded that superovulatory treatment with 1000 IU PMSG plus AP administered at a fixed time after the onset of estrus may improve ovarian response and the yield of viable embryos in ewes.     

Ovarian features contributing to the variability of PMSG-induced ovulation rate in sheep

In Exp. 1, ovulation rate was measured in three groups of Romanov ewes given two injections of 600 i.u. PMSG 3 weeks apart with the ewes intact (Group I, N = 8), a similar treatment with the ewes intact at the first injection and unilaterally ovariectomized at the second (Group II, N = 8), or unstimulated ewes which were hemispayed at the same time as Group II ewes (Group III, N = 6). In Exp. 2, the follicular population of one ovary was correlated with the number of ovulations induced by 600 i.u. PMSG in the contralateral ovary (10 Romanov ewes). From 8.4 +/- 1.8 (Group I) and 8.2 +/- 3.3 (Group II) CL at the first injection, PMSG-induced ovulation rate at the second injection decreased to 3.9 +/- 1.8 and 3.7 +/- 1.2 in Groups I and II respectively, a value similar for ewes with 1 or 2 ovaries. Furthermore, despite no major changes in the number of antral follicles after the first injection, there was no correlation (r = -0.09) between the response to the two successive injections in intact ewes. Comparison of the ovarian status of the ovary removed before the PMSG injection (Group II ewes of Exp. 1, ewes of Exp. 2) to the number of CL found in the remaining ovary demonstrated that PMSG-induced ovulation rate was not correlated with the overall antral follicle population (r = 0.62 in Exp. 1, r = 0.49 in Exp. 2).(ABSTRACT TRUNCATED AT 250 WORDS)     

Oestrus, time of ovulation, ovulation rate and conception rate in progestagen-treated ewes given Gn-RH, Gn-TH analogues and gonadotrophins.

The influence of Gn-RH, hCG and a PMSG-hCG mixture (PG600) on the time of ovulation, ovulation rate and on the occurrence of oestrus in ewes treated with progestagen-impregnated sponges for 12 days examined. The effects of Gn-RH analogues on plasma LH, oestrus, ovulation and conception rate were also investigated. Six separate experiments were carried out. When 50 micrograms Gn-RH were given 24 h after sponge removal ovulation occurred in 44--46% of ewes within 24 h and in all ewes by 34 h. Gn-RH was a more potent ovulation synchronizer than hCG. Both hCG and PG600 reduced the incidence of overt oestrus. Gn-RH also had this effect in ewes treated during February and May but not in August and September. Gn-RH analogues given 2 days before sponge removal significantly increased ovulation rate. The display of oestrus was not affected in ewes treated 2 days before sponge removal but was suppressed in 43-69% of ewes treated with an analogue at the time of sponge removal. Ovulation occurred in 50-62% of ewes within 30-35 h of injection of Gn-RH analogues, regardless of the time of their administration. The release of LH in response to one analogue was not influenced by the presence of the progestagen-impregnated sponge in the vagina. When given a Gn-RH analogue 2 days before sponge removal or at the time of sponge removal 63 and 62% of mated ewes became pregnant compared with 70% of control ewes.     

Induction of oestrus and fertility in the anoestrous ewe with hormones and controlled lighting and temperature

A programme consisting of 14 daily injections of progesterone (10 mg) followed by single injection of PMSG (500 i.u.) and oestradiol- 17 beta (30 micrograms), along with controlled temperature (18-20 degrees C) and lighting (10 h light/24 h), was applied to 60 anoestrous ewes between late May and early August to induce reproductive activity. Breeding started within 24 h of the oestradiol injection and 80.0% of the ewes conceived at the induced oestrus. Dorset ewes had higher conception (95.2 versus 71.8%) and prolificacy (1.74 versus 1.52 fetuses/ewe) rates than did crossbred Suffolk ewes. Plasma progesterone concentrations during progesterone administration were significantly higher than those found during anoestrus and were generally lower in ewes which did not conceive than in those which did. The plasma progesterone data indicated that ovulation had occurred in most of the ewes which were not pregnant at 90-100 days and that many may have been pregnant initially but then lost the conceptus.     

Electromyographic activity of the uterus in the ewe during the sexual season: comparison of natural estrus and estrus induced by progestagens alone or with PMSG supplementation

Electromyographic (EMG) activity of the uterus was recorded in vivo in 3 groups of ewes in oestrus during the breeding season. The ewes were either in natural oestrus or in oestrus induced by progestagens with or without PMSG supplementation. Plasma concentrations of LH were measured at regular intervals in order to determine the onset of the preovulatory surge of LH (To). During natural oestrus, the uterus exhibited a spontaneous rhythmic activity composed of bursts of potentials. Burst frequency was maximal at the moment of preovulatory LH release, then decreased. Percentages of downward, upward and non-definite propagations were calculated during 1-h periods at the beginning of the LH peak and 6, 12, 24 and 36 h afterwards. There was no difference between the relative proportions of these types of propagation at any of the times analysed. Mean burst frequency was not modified by progestagen pre-treatment but showed more interanimal variability at the moment of maximal frequency in ewes treated with progestagen. For the first 12 h after To, the organization of uterine activity was not modified by progestagens but non-definite propagation became predominant 24 and 36 h after To, namely from the time of ovulation onward. PMSG increased burst frequency. The increase of frequency variability between animals, normally observed at the beginning of oestrus, and the predominance of non-definite propagation, normally observed at the end of progestagen-induced oestrus, did not appear when the ewes had been treated with PMSG.     

Effect of immunization on reproducvive performance, embryo quality and progesterone in Rasa Aragonesa ewes actively immunized against androstenedione or passively immunized against testosterone

A total of 217 Rasa Aragonesa ewes were used to test two immunization treatments: 1.Active immunization against androstenedione: ewes immunized in previous matings (androstenedione, reimmunized; AR groups, n=58) or not (first immunization; AF groups n=64) were boosted either 2 or 4 wk before mating. 2.Passive immunization against testosterone: antisera were injected either at sponge withdrawal (zero time; T0 group, n=21) or 1 wk previously (Tl group, n=22). We used 52 ewes as controls (C group). Half of each group was used either to record reproductive performance or to embryo viability assessment. Prolificacy was significantly increased in ewes which reached a moderate antibody level, independently of the treatment. Fertility was lower in AR ewes that attained a high antibody titre (P<0.01). The percentage of viable embryos recovered was lower in AF ewes (P<0.01), and in ewes whose testosterone antibody titre was high (P<0.05), compared to C group. It was proven that similar or lower antibody levels were more harmful for ewes from AF and Tl than for ewes from AR or T0 groups. The proportion of nonfertilized recovered ova was not significant. Progesterone levels were notably increased in AR ewes (P<0.001) independently of ovulation rate and were positively correlated to antibody titre at mating (P<0.01) but these events were not observed in T ewes. These findings indicate that after androgen immunoneutralization, only those ewes having antibody titres within a limited range at mating had improved reproductive performance. Further research is needed in order to understand the role that progesterone plays in immunized ewes.     

Progesterone priming and age of ewe affect the life-span of corpora lutea induced in the seasonally anovulatory Merino ewe by the 'ram effect'

Increasing doses of progesterone from 0 to 20 mg, given as a single i.m. injection, increased the proportion of corpora lutea that had a normal life-span when induced in ewes by the introduction of testosterone-treated wethers from 54% (19/35) to 100% (34/34). Injection of progesterone did not affect the induction of ovulation and 95% (130/136) of the anovulatory ewes ovulated. Nevertheless, a low proportion of ewes displayed oestrus between Days 16 and 26 after the introduction of testosterone-treated wethers (Exp. 1, 47%, 92/196; Exp. 2, 50%, 502/1000). Many of the ewes that did not display oestrus also failed to ovulate again (Exp. 1, 70%, 37/53). The proportions of anovulatory adult and maiden ewes that ovulated after the introduction of testosterone-treated wethers were not significantly different but significantly fewer maiden ewes were detected in oestrus.     

Plasma hormone levels and reproductive behaviour in anoestrous ewes after treatment with progesterone and PMSG

Plasma progesterone and gonadotrophin levels were studied in anoestrous ewes treated during June or July with a subcutaneous progesterone implant and/or an injection of oestradiol or PMSG. Of 32 ewes treated with progesterone during July, 9 showed a gonadotrophin surge after removal of the implant, and 10 ewes showed oestrous behaviour during the following 4 days. Six ewes conceived at this induced oestrous. Progesterone treatment during June was much less effective, with only 2 of 19 treated ewes showing a gonadotrophin surg and oestrous behaviour. Administration of PMSG at the time of implant removal in the June experiment was followed by a gonadotrophin surge and oestrous behaviour in 18 of 19 ewes, and 15 ewes conceived at the induced oestrus. An injection of PMSG, without progesterone pretreatment, stimulated a gonadotrophin surge and ovulation, but did not result in oestrous behaviour. The treatments employed appeared to initiate cyclic ovarian activity in the July experiment, but not in the June experiment.     

Time of ovulation in goats (Capra hircus) induced to superovulate with PMSG

The timing of ovulation in feral goats treated with 1200 i.u. PMSG +/- 50 micrograms GnRH was studied by repeated laparoscopy. Experiment 1 established that superovulation began as early as 30 h after withdrawal of progestagen-impregnated sponges and was not completed at 54 h if goats received PMSG alone. GnRH synchronized ovulation, leading to 91% of ovulations appearing between 36 and 48 h after sponges were withdrawn. Experiment 2 established that superovulation continued until up to 77 h in goats treated only with PMSG. The stress of repeated laparoscopy appeared to delay or abolish ovulation in some females. The mean (+/- s.e.) ovulation rate was greater in goats treated with GnRH (12.7 +/- 1.3) than in those that received PMSG only (9.7 +/- 1.1; P less than 0.05). Out of 47 of the females in Exp. 1, 43 had one or more corpora lutea at laparoscopy 24 h after withdrawal of progestagen. These early corpora lutea were associated with an increased concentration of plasma progesterone during the periovulatory period. Experiment 3 provided evidence that these corpora lutea arose before the withdrawal of progestagen-impregnated sponges.