Time course of the luteinizing hormone surge in cannulated rats: quantitative variance estimates within individual rats over successive cycles

The present study was designed to compare quantitative variance estimates in the profile of the luteinizing hormone (LH) surge within individual rats over successive proestrous (PE) days (WITHIN) with the variability between rats (BETWEEN). Sprague-Dawley female rats were implanted under ether anesthesia with indwelling intracardiac cannulas. On successive PE afternoons of normal 4-day cycles, hourly blood samples (0.25 ml) were collected via the cannula from 1400-2000 h for radioimmunoassay of plasma LH. Three characteristics which reflected the profile of the LH surge were examined: the time of onset of LH release, the time of peak LH release, and the magnitude of peak LH release. Twenty-one animals yielded LH surge data on a total of 42 PE days with a mean (+/- SD) time of onset = 1534 h +/- 66 min, time of peak = 1730 h +/- 75 min, and magnitude of peak = 1176 +/- 441 ng NIAMDD-Rat LH-RP-1/ml plasma. Variance estimates BETWEEN and WITHIN animals were determined by analysis of variance and the method of Vaughan and Corballis (1969) for calculating percent of total variance. Differences BETWEEN in time of onset of LH release approached significance (P = 0.05-0.10) and contributed 50.5% of the total variance compared to a negative value for differences WITHIN. Differences BETWEEN in time of peak LH release were significant (P less than 0.05) and contributed 51.5% of total variance compared to a negative value for differences WITHIN. In contrast, for the magnitude of peak LH release, neither differences BETWEEN nor WITHIN contributed substantially to total variance (both negative values), with the major contribution from the residual term.(ABSTRACT TRUNCATED AT 250 WORDS)     

Urinary hormone analysis as a diagnostic tool to evaluate the ovarian function of female gorillas (Gorilla gorilla)

Daily urine samples were collected from 4 adult female gorillas over 7 menstrual cycles. Urinary oestrone conjugate and pregnanediol-3-glucuronide (PDG) were measured by radioimmunoassay; LH was measured by enzyme immunoassay and each hormone was indexed by creatinine. The quantity of urinary LH during the ovulatory surge was positively correlated with the quantity of PDG excreted during the luteal phase (r = 0.87, P = 0.0013). The observations indicate a relationship between the quality of the LH surge and the levels of PDG in the luteal phase and suggest that both the LH surge and subsequent luteal phase function may be predictable from the oestrogen excretion profile during the follicular phase.     

Luteinizing hormone concentrations in anestrous ewes administered various estrogens

Twenty four anestrous ewes were evenly assigned to one of six groups and administered either sesame oil, estradiol-17β, estradiol-17α, estrone, estradiol benzoate or estradiol valerate. All estrogen treated ewes received 50 μg of the respective estrogen. Blood plasma was collected for 28 hours post-treatment and quantified for luteinizing hormone (LH) by radioimmunoassay. An estrogen induced LH surge was detected in at least three of the four ewes administered either estradiol-17β, estrone, estradiol benzoate or estradiol valerate whereas only one of the four estradiol-17α treated ewes and none of the ewes administered sesame oil had an LH surge. The interval from treatment to peak LH was similar for estradiol-17β (17.3±2.7 hours), estrone (18.5±1.0 hours) and estradiol benzoate (19.0±0.6 hours) treated ewes but delayed 7 to 9 hours for ewes administered estradiol valerate (26.0±1.2 hours).     

A semiquantitative and rapid radioimmunosorbent assay of luteinizing hormone for early prediction of time of ovulation in cattle

A radioimmunoassay (RIA) for bovine luteinizing hormone (LH) is presented. It is based on the binding of the second antibody to polystyrene vials. Less than 4 h is required to estimate the hormone level in the plasma. LH content is determined by comparing unknown plasma samples to relevant control plasma to identify the preovulatory LH surge. Of 182 samples, 176 (97%) were found to contain either more or less LH than the control plasma, and 12 out of 13 (92%) LH surges were identified using the immunosorbent RIA. The method can be used for early prediction of ovulation in cattle, and it may be used to time aspiration of oocytes.     

Temporal relations between plasma concentrations of luteinizing hormone, follicle-stimulating hormone, estradiol-17beta, progesterone, prolactin, and alpha-melanocyte-stimulating hormone during the follicular, ovulatory, and early luteal phase in the bitch

Compared with other domestic animals, relatively little is known about the changes in, and temporal relations between, reproductive hormones around the time of ovulation in the domestic bitch. Therefore, plasma concentrations of luteinizing hormone (LH), follicle-stimulating hormone (FSH), estradiol-17beta, progesterone, prolactin (PRL), and alpha-melanocyte-stimulating hormone (alpha-MSH) were determined one to six times daily from the start of the follicular phase until 5 days after the estimated day of ovulation in six Beagle bitches. In all bitches, the pre-ovulatory LH surge was accompanied by a pre-ovulatory FSH surge. A pre-ovulatory PRL or alpha-MSH surge was not observed. The pre-ovulatory FSH and LH surges started concomitantly in four bitches, but in two bitches the FSH surge started 12 h earlier than the LH surge. The FSH surge (110+/-8 h) lasted significantly longer than the LH surge (36+/-5 h). In contrast with the pre-ovulatory FSH surge, the pre-ovulatory LH surge was bifurcated in four of six bitches. The mean plasma LH concentrations before (1.9+/-0.4 microg/L) and after (1.9+/-0.3 microg/L) the LH surge were similar, but the mean plasma FSH concentration before the FSH surge (1.6+/-0.3 U/L) was significantly lower than that after the FSH surge (3.1+/-0.2 U/L). In most bitches the highest plasma estradiol-17beta concentration coincided with or followed the start of the pre-ovulatory LH surge. In five of the six bitches the plasma progesterone concentration started to rise just before or concurrently with the start of the LH surge. In conclusion, the results of this study provide evidence for the differential regulation of the secretion of LH and FSH in the bitch. In addition, the interrelationship of the plasma profiles of estradiol-17beta and LH suggests a positive feedback effect of estradiol-17beta on LH surge release. The start of the pre-ovulatory LH surge is associated with an increase in the plasma progesterone concentration in this species.     

Exogenous GnRH induces ovulation in seasonally anoestrous lactating goats (Capra hircus)

A specific sheep LH radioimmunoassay was validated for the measurement of goat LH, and used to monitor luteal-phase LH episodes and the preavulatory LH surge in progestagen sponge-synchronized cycling goats. No luteal-phase LH episodes were detected during 12 h of frequent (15-min) blood sampling in 2 goats. A preovulatory LH surge was recorded in 5/5 goats, with a mean amplitude of 45.4 +/- 7.2 ng/ml and a mean time of onset of 38.4 +/- 1.2 h after removal of a progestagen-impregnated sponge. In anoestrous goats, single i.v. injections of 1000 and 2000 ng GnRH induced LH episodes with a mean amplitude of 2.04 +/- 0.11 and 3.67 +/- 0.06 ng/ml respectively, but injections of 250 or 500 ng did not consistently elevate LH concentrations. Progestagen-primed, seasonally anoestrous lactating goats were treated with repeated injections of 1500 ng GnRH (every 2 h for 52 or 78 h) in May 1985 or 1986. All 10 had kidded in March of the same year, and were consequently at peak lactation at the time of GnRH treatment. A preovulatory LH surge was detected in 9 goats with a mean time of onset of 59.5 +/- 2.9 h (1985) or 39.6 +/- 3.3 h (1986) after vaginal sponge removal. All animals displayed oestrus and ovulated, and 9 of the goats were mated: in 5 of these animals pregnancies were successfully carried to term. The results show episodic LH release in response to GnRH and indicate that ovulation can be induced in seasonally anoestrous goats, even at peak lactation, and normal pregnancies may result.     

Increased fertility in rhesus monkeys by breeding after the preovulatory LH surge.

Fertility in a rhesus monkey breeding colony was significantly increased by caging females with males immediately after detection of the preovulatory luteinizing hormone (LH) surge. LH was measured in daily serum samples by a rapid (24-hr) radioimmunoassay which used iodinated ovine LK as tracer and an antiserum to human LH.     

The relationship between electrical resistance of vaginal mucus and plasma hormonal parameters during periestrus in sows

Sixteen crossbred multiparous sows displaying estrus on Day 5 or 6 after weaning were used in this study. Jugular veins of sows were cannulated on Day 13 of the estrous cycle. Electrical resistance of the vaginal mucosa was measured twice daily on Days 17 to 19 of the cycle and at 4-h intervals (excluding 3 a.m.) during the periestrous period. Blood was sampled every 4 h beginning on Day 17 and continuing for 6 to 7 d. Blood samples were assayed for LH, P₄, E₂, androstenedione (A₄) and testosterone (T) by radioimmunoassay. All data were standardized to maximum LH concentration (0 h). The mean LH surge lasted about 28 h and its mean amplitude was 6.5 + 0.8 ng/ml of plasma. Vaginal electrical resistance (VER) decreased 4 d before the LH peak, remained low for 3 d and gradually started to increase after 0 h. The first signs of estrus were observed 16.9 + 17.8 h prior to the LH peak. The range of the interval was −44 h to +8 h. The increase in VER followed peak LH by 6.2 + 4.5 h. Intervals from peak LH to the beginning of the VER increase ranged from 0 to 16 h. Variation of the interval from the onset of estrus to the LH peak was significantly higher than that of the interval from LH peak to the beginning of the increase in VER (P < 0.005). The decrease in the VER observed during the follicular phase coincided with low levels of P₄ (<1 ng/ml) and increasing concentrations of E₂. Profiles of E₂ and both androgens (A₄ and T) were similar; these hormones increased gradually during the follicular phase of the cycle. The highest values of E₂, A₄ and T were observed before and during the first hours of the preovulatory LH surge. Sows with ovarian cysts (n = 3) had atypical patterns of electrical resistance and aberrant plasma hormone concentrations. These results indicate that measurement of VER can be utilized for detection of LH surges during estrus in sows. Moreover, the monitoring of VER changes provides a more reliable indication of the LH surge than detection of estrus.     

Timing of ovulation in relation to onset of estrus and LH peak in yak (Poephagus grunniens L.)

The objective of the study was to determine the timing of ovulation in relation to onset of estrus and the preovulatory LH peak in yaks. For this purpose, a sensitive LH enzymeimmunoassay previously established in buffaloes was successfully validated for measuring the hormone in yak plasma. Plasma LH and progesterone were estimated from blood samples collected from eight non-lactating cycling yaks at 2 h intervals after estrus onset until 6 h after ovulation (ovulation was confirmed by palpation of ovaries per rectum). The mean+/-S.E.M. preovulatory plasma LH peak was 10.11+/-0.35 ng/ml with the values ranging from 8.75 to 11.51 ng/ml in individual yaks. The mean+/-S.E.M. duration of the LH surge was 7.25+/-0.55 h with a range of 6-10 h. Onset of LH surge (mean+/-S.E.M.) occurred 3.0+/-0.65 h after the onset of estrus. Mean plasma progesterone stayed low (<0.25 ng/ml) during the entire duration of sampling. Ovulation occurred 30.5+/-0.82 h (range, 28-34 h) after the onset of estrus and 20.25+/-1.03 h after the end of LH surge. The occurrence of the LH peaks within a narrow time frame of 4-8h post estrus onset in yaks could have contributed to the animals ovulating within a narrow time interval.     

Estrogen and LH dynamics during the follicular phase of the estrous cycle in the Asian elephant

Pituitary and corpus luteum hormone patterns throughout the elephant estrous cycle have been well characterized. By contrast, analysis of follicular maturation by measurement of circulating estrogens has been uninformative. This study tested the ability of a urinary estradiol‐3‐glucuronide radioimmunoassay to noninvasively assess follicular development during the nonluteal phase of the elephant estrous cycle, and to determine the relationship between estrogen production and the “double LH surge.” Daily urine and serum samples were collected throughout seven estrous cycles from three Asian elephants, and urine was collected from an additional three females, for a total of 13 cycles. Serum was analyzed for luteinizing hormone (LH), and urine was analyzed for estrogens and progestins. Elephants exhibited a typical LH pattern, with an anovulatory LH (anLH) surge occurring approximately 21 days before the ovulatory LH (ovLH) surge. The urinary estrogen pattern indicated the presence of two follicular waves during the nonluteal phase. The first wave (anovulatory) began 5 days before the anLH surge and reached a maximum concentration the day before the peak. Thereafter, urinary estrogens declined to baseline for 2 weeks before increasing again to peak concentrations on the day of the ovLH surge. Urinary progestins were baseline throughout most of the follicular phase, increasing 2–3 days before the ovLH surge and continuing into the luteal phase. These results support previous ultrasound observations that two waves of follicular growth occur during the nonluteal phase of the elephant estrous cycle. Each wave is associated with an increase in estrogen production that stimulates an LH surge. Thus, in contrast to serum analyses, urinary estrogen monitoring appears to be a reliable method for characterizing follicular activity in the elephant. Zoo Biol 22:443–454, 2003. © 2003 Wiley‐Liss, Inc.     

The acute effects of oestradiol-17beta and synthetic LH-RH on plasma LH levels in freemartin heifers.

Injection of oestradiol was followed by a surge of plasma LH within 24 h in only 7 of 12 freemartins. Elevations of plasma LH were less than those reported for normal non-cyclic heifers, but some freemartins showed a delayed, or more prolonged, LH response. Responsiveness to oestradiol was not related to degree of chimaerism or plasma androstenedione level, and most of the animals responded similarly in two trials carried out 4 months apart, during which time plasma androstenedione levels had more than doubled. Freemartins which showed an LH surge after oestradiol treatment released greater amounts of LH after the injection of LH-RH than did non-responders.     

Effect of nembutal on LH release in baboons.

Regularly cycling female baboons were selected and maintained under a diurnal light schedule from 0500 to 1900 hr (CST). Beginning three days prior to the expected LH peak, blood was collected daily at 0800 and 1600 hr for 6 days in 5 baboons under light sedation for radioimmunoassay of plasma LH and estrogen. The plasma level of LH increased linearly and reached a peak in the afternoon of the second day. The peak in plasma estrogen appeared prior to the LH peak. In order to examine the critical period of LH surge in baboons, nembutal was injected daily at 1300 hr beginning a few days prior to expected LH relase. Initial dose of nembutal was 35 mg/kg body weight, but a supplementary dose was later required for a full 5 hours of anesthesia. Blood was collected at 1600 hr from 4 baboons during nembutal injections and after cessation of nembutal injections for radioimmunoassay of plasma LH and estrogen. It was found that nembutal injections suppressed LH release in 2 baboons, and caused a delay of LH release in 2 baboons. However, the plasma level of estrogen declined immediately after initiation of nembutal injection and remained lower. The evidence illustrates the nature of the neural components of LH release which became effective in the afternoon during the ovulatory phase. In addition, a linear increase in plasma level of LH, which is due to accumulation of circulating LH, is necessary for induction of ovulation in baboons.     

Coital stimuli controlling luteinizing hormone secretion and ovulation in the female ferret

A series of experiments focused on the masculine coital behaviors controlling pituitary luteinizing hormone (LH) secretion and reflex ovulation in the estrous female ferret. An initial experiment investigated which coital stimuli from the male are required to induce ovulation. It was found that corpus luteum formation, which served as an index of ovulation, occurred in estrous female ferrets only if the male achieved a penile intromission. Neck gripping, mounting, and pelvic thrusting behavior without intromission by the male failed to induce ovulation. A second experiment investigated the timing and magnitude of the coitus-induced LH surge associated with ovulation. Blood was obtained via jugular catheters from estrous females in various mating situations. Plasma LH concentrations were measured by a heterologous radioimmunoassay that was validated for use in the ferret. A significant surge in plasma LH occurred only when an intromission was achieved by the stud male. Plasma LH was significantly elevated 2.0 h after the introduction of the male, peak values were reached 6.0 h later, and this elevation lasted on average 5.7 hours (5/5 females). No LH rise occurred in 2/2 female ferrets in which only neck gripping, mounting, and pelvic thrusting, but no intromission, were allowed to occur. The ferret mating pattern and the resultant LH response differ from those seen in three other induced ovulators (cat, vole, and rabbit) in which the male's intromission latency and duration are much shorter than in the ferret, and in which a distinctive peak in plasma LH often occurs within 1 h after mating.     

Serum luteinizing hormone levels in Brangus cows following variable suckling intensity and administration of various levels of estrogen

Fifty Brangus cows were randomly allotted to suckled (S) or nonsuckled (NS) treatment groups on day 20 postpartum. Suckled cows were nursed at 6 hr intervals for 72 hours. Nonsuckled cows were separated from their calves for the entire 72 hours. At 24 hr after initial separation from calves, S and NS cows were given an I.M. challenge of 0, 0.5, 1.0, 2.0 or 4.0 mg estradiol-17beta (E2) to induce a luteinizing hormone (LH) surge (five cows per treatment group). Blood samples were taken at the time of E2 injection and at 2 hr intervals until hr 48 post-injection. Blood serum was analyzed for LH content via radioimmunoassay. Suckled and NS cows manifesting an LH surge after receiving less than 4 mg E2 were 2 of 15 vs 9 of 15 (P<.01), or 4 mg E2 dose were 5 of 5 vs 5 of 5, respectively. Greater serum LH concentrations in NS than S cows were found with dose levels of 0, 0.5 and 1.0 mg E2 (P<.005), but there was no difference by period. Differences by treatment (P<.05) and by period (P<.005) were found at the 2 mg E2 dose. Suckled and NS cows having an LH surge at less than a 4 mg E2 challenge had no differences in LH concentration or timing parameters. Four mg E2 hastened the time of onset of the LH surge (P<.025), time till peak height of the surge (P<.025) and completion of the surge (P<.10). No differences in postpartum interval or conception rate were found between S and nonsuckled. Suckling impairs hypothalamic/pituitary response to low E2 challenge dose and elicits changes in timing parameters in response to high E2 dosage.     

Gonadotrophin secretion during the periovulatory period in Galway and Finnish Landrace ewes and Finnish Landrace ewes selected for high ovulation rate

Rates of ovulation differed significantly (P less than 0.01) among ewes of the different genetic lines. However, of the reproductive characteristics studied, only progesterone concentration at the height of luteal function, duration of oestrus, and interval from onset of oestrus to peak of the preovulatory gonadotrophin surge showed significant positive association with rate of ovulation. The pattern of secretion of LH during the periovulatory period did not differ in the Galway and Finnish Landrace breeds. The total amount of LH secreted during the preovulatory surge did not differ amongst lines. Similarly, no difference in the plasma concentration of LH at the height of the preovulatory surge was noted among Galway and reference Finnish Landrace lines. However, the concentration of LH at the height of the surge was significantly (P less than 0.05) reduced in the selected Finnish Landrace line. Plasma concentrations of FSH during the preovulatory period were significantly (P less than 0.05) elevated in the breed (Galway) with the lowest prolifcacy. When contrasted with either of the Finnish Landrace lines, the magnitudes of the preovulatory surge of FSH and the secondary surge of FSH were significantly greater (P less than 0.05) in Galway ewes. These results suggest that genetic difference in rate of ovulation among sheep breeds is not tightly coupled to quantitative differences in plasma concentration of gonadotrophic hormones during the periovulatory period.     

Differential regulation of preovulatory luteinizing hormone and follicle-stimulating hormone release by opioids in the proestrous rat

We have investigated the role of mu- and kappa-opioid receptors in the central control of preovulatory LH and FSH release in the proestrous rat. Animals were anesthetized with chloral hydrate at 14:00 h on proestrus day. Following femoral artery cannulation, they were mounted in a stereotaxic apparatus. Morphine and U-50488H (benzene-acetamide methane sulphonate) were infused intracerebroventricularly either alone or in combination with naloxone and MR1452, respectively. Controls received sterile saline alone. Blood samples were obtained at hourly intervals between 15:00 h and 17:00 h. Plasma LH and FSH levels were measured by radioimmunoassay. Morphine did not significantly change plasma LH levels at 15:00 h and 16:00 h sampling intervals. A significant increase was observed at 17:00 h compared to the controls (p<0.05). U-50488H significantly increased LH levels at 16:00 h and 17:00 h (p<0.05). The co-administration of naloxone and MR1452 with mu- and kappa-agonist had no significant effect on LH levels at any sampling interval. In all groups, LH levels showed a linear rise over the sampling period between 15:00 h and 17:00 h. None of the treatments significantly altered plasma FSH levels which however, declined towards the end of the afternoon surge. In conclusion, we suggest that the secretion of LH and FSH is differentially regulated by mu- and kappa-opioid receptors. It is thought that in all groups chloral hydrate interfered with the LH surge secretory systems.     

Novel wheel running blocks the preovulatory luteinizing hormone surge and advances the hamster circadian pacemaker

In rodents, the preovulatory luteinizing hormone (LH) surge is timed by a circadian rhythm. We recently reported that a phenobarbital-induced delay of the estrous cycle in Syrian hamsters is associated with an approximately 2-h phase advance in both the circadian locomotor activity rhythm and the timing of the LH surge. The following study tests the hypothesis that a >2-h nonpharmacological phase advance in the circadian pacemaker that delays the estrous cycle by a day will also phase advance the LH surge by approximately 2 h. Activity rhythms were continuously monitored in regularly cycling hamsters using running wheels or infrared detectors for about 10 days prior to jugular cannulation. The next day, on proestrus, hamsters were transferred to the laboratory for 1 of 3 treatments: transfer to a "new cage" (and wheel) from zeitgeber time (ZT) 4 to 8 (with ZT12 defined as time of lights-off), or exposure to a "novel wheel" at ZT5 or ZT1. All animals were then placed in constant dark (DD). Blood samples were obtained just before onset of DD and hourly for the next 6 h, on that day and the next day for determination of plasma LH concentrations. Running activity was monitored in DD for about 10 more days. Transfer to a novel wheel at either ZT5 or ZT1 delayed the LH surge to day 2 in most hamsters, whereas exposure to a new cage did not. Only the delayed LH surges were phase advanced at least 2.5 h on average in all 3 groups. However, wheel-running activity was similarly phase advanced in all 3 groups regardless of the timing of the LH surge; thus, the phase advances in circadian activity rhythms were not associated with the 1-day delay of the LH surge. Interestingly, the number of wheel revolutions was closely associated with the 1-day delay of LH surges following exposure to a novel wheel at either ZT1 or ZT5. These results suggest that the intensity of wheel running (or an associated stimulus) plays an important role in the circadian timing mechanism for the LH surge.     

Comparison of the rate of decline in plasma progesterone concentrations at a natural and progesterone-synchronized oestrus and its effect on tonic LH secretion in the ewe

The pattern of change in plasma progesterone and LH concentrations was monitored in Clun Forest ewes at a natural oestrus and compared to that observed after removal of progesterone implants. The rate of decline in plasma progesterone concentrations after implant withdrawal (1.8 +/- 0.2 ng/ml h-1) was significantly greater (P less than 0.001) than that observed at natural luteolysis (0.2 +/- 0.1 ng/ml h-1), and this resulted in an abnormal pattern of change in tonic LH secretion up to the time of the preovulatory LH surge. This more rapid rate of progesterone removal was also associated with a shortening of the intervals from the time that progesterone concentrations attained basal values to the onset of oestrus (P less than 0.05) and the onset of the preovulatory LH surge (P less than 0.01). However, there were no significant differences in the duration of the LH peak, preovulatory peak LH concentration, ovulation rate or the pattern of progesterone concentrations in the subsequent cycle. It is suggested that the abnormal patterns of change in progesterone and tonic LH concentrations may be one factor involved in the impairment of sperm transport and abnormal patterns of oestradiol secretion known to occur at a synchronized oestrus.     

Effects of supraphysiological concentrations of progesterone on the characteristics of the oestradiol-induced gonadotrophin surge in women

Intramuscular injections of oestradiol benzoate were given to 8 normally cyclic women in the early follicular phase of 3 different cycles. Progesterone was also injected in the second (low dose) and the third cycle (high dose). Oestradiol induced simultaneous surges of LH and FSH in all women and the onset of these surges was advanced by progesterone. Low-dose progesterone induced a significant increase in the amplitude and the duration of the LH and FSH surges, while high-dose progesterone decreased the duration significantly. In contrast to the oestrogen-only treatment cycles, when the women were treated with progesterone, basal LH and FSH concentrations were suppressed significantly not only before the onset but also after the end of the surge. The results suggest that progesterone affects the dimension of the oestradiol-induced gonadotrophin surge by exerting both a stimulatory and an inhibitory effect on pituitary gonadotrophin secretion. Supraphysiological concentrations of progesterone decreased the duration of the oestradiol-induced gonadotrophin surge significantly and this is possibly part of the mechanism which attenuates the endogenous LH surge in women superovulated for in-vitro fertilization.     

Seasonal variation in preovulatory events associated with synchronization of estrus in dwarf goats

This experiment was conducted to define the temporal relationships among estrus, the LH surge and ovulation after estrus synchronization in dwarf goats and to assess the effect of season on these parameters. In November (breeding season), March (transition period) and July (non-breeding season), estrus was synchronized in 12 dwarf goats by means of intravaginal sponges containing 60 mg medroxyprogesterone acetate (MAP) for 10 d, coupled with 125 microg cloprostenol i.m. 48 h before sponge removal and 300 IU eCG i.m. at sponge removal. A different group of animals was used during each time period. Onset of estrus was monitored using two males, and blood samples for the measurement of plasma LH were collected at 2-h intervals from 24 to 60 h after sponge removal. Ovulation was confirmed by laparoscopy at 54 and 72 h after sponge removal. A seasonal shift was detected in the intervals to onset of estrus, LH surge, and ovulation after sponge removal (P<0.05), with sponge removal to onset of estrus being shorter (P<0.05) in November (25.0 +/- 1.56 h) and July (28.9 +/- 2.43 h) than in March (40.9 +/- 3.27 h). The intervals between onset of estrus and the LH surge and between the LH surge and ovulation were found to be constant throughout the different seasons. An optimal time for breeding, artificial insemination, oocyte and embryo recovery, and embryo transfer may be predicted using information gained from these studies.