Effects of naloxone and animal temperament on serum luteinizing hormone and cortisol concentrations in seasonally anestrous Brahman heifers

The effect of endogenous opioid peptides (EOP) and individual animal temperament on serum luteinizing hormone (LH) were investigated in seasonally anestrous Brahman heifers (n = 24). Animals that had shown behavioral estrus in previous months but that had not returned to estrus for at least 30 d were selected. The heifers were ranked by temperament (tame = 1, normal = 2, wild = 3) and randomly allotted into three groups. Blood was collected from one heifer of each group per day. Blood samples were taken via jugular cannula every 15 min for 6 h and every 30 min for another 4 h. After the first hour of sampling, the heifers received intravenous saline (SAL, n = 8); naloxone (LN, 0.5 mg/kg i.v., n = 8); or naloxone (HN, 1.0 mg/kg i.v., n = 8). Three hours after naloxone treatment, each heifer was given gonadotropin releasing hormone (GnRH, 100 mug i.m.). All samples were processed to yield serum and were assayed for LH by radioimmunoassay (RIA). Hourly samples were assayed for cortisol by RIA. The area under the LH curve 60 min postnaloxone treatment was higher in LN and HN than in SAL (57.0 and 40.8 vs 6.1 units; P<0.01); and the area under the 180 min postnaloxone curve remained higher in LN than in SAL (106.2 vs 35.1 units; P<0.05). Cortisol concentrations 60 min postnaloxone administration were above prenaloxone levels(38.2 vs 26.7 ng/ml; P<0.0002). Temperament scores of heifers were positively correlated with cortisol release. The area under the cortisol curve had a negative correlation with mean LH. Serum LH concentrations appear to be suppressed by EOP in seasonally anestrous Brahman heifers, and EOP appear to reduce serum cortisol concentrations. Excitable heifers had higher concentrations of serum cortisol, which negatively affected serum LH concentrations.     

Steroid hormone feedback on LH in prepubertal Holstein heifers

A significant dose-response relationship between gonadotropin-releasing hormone (GnRH) and time to luteinizing hormone (LH) peak, peak serum LH and total serum LH was obtained in prepubertal Holstein heifers (28 weeks of age) (Experiment 1). For the second experiment, the effect of steroid feedback on the anterior pituitary was determined. A steady infusion of saline, estradiol-17 beta or progesterone was maintained for 24 h while GnRH, in various schemes, was administered 8 h after the beginning of steroid infusion. Estradiol-17 beta infusion (2.08 micrograms/h), although it did not affect peripheral concentrations of estrogen, caused an LH release 24 to 30 h later in 37.5% of the heifers. This amount of exogenous estrogen did not affect the LH response to a single GnRH (4 micrograms) challenge. When the same GnRH dosage (4 micrograms) was administered 6 times at hourly intervals, the heifers infused with estradiol had a lower response after the first 2 injections of GnRH and a greater response after the last 4 injections than heifers infused with saline. When GnRH was infused (4 micrograms/h) for 6 h, beginning 8 h after steroid infusion, estradiol infusion caused a significantly higher peak LH and total LH release than an infusion of either saline or progesterone (7.3 micrograms/h). The progesterone infusion had no effect on the GnRH-stimulated LH release. We conclude that prepubertal dairy heifers have an anterior pituitary capable of responding to the feedback effect of estrogen in a positive manner.     

Effects of circulating progesterone and insulin on early embryo development in beef heifers

The aims of this study were to determine the effect on early embryo development of feeding a diet formulated to enhance circulating insulin concentrations and secondly to investigate the association between early embryo development and maternal progesterone concentrations in beef heifers. The study was carried out in 32 Simmental x Holstein Friesian heifers 22-25 months of age weighing 506+/-7kg and in condition score 3.1+/-0.1. Animals were fed two diets that were isoenergetic and isonitrogenous, but that would encourage either propionate (diet A) or acetate (diet B) production in the rumen. The rationale was that propionate would induce a greater insulin release in response to feeding. Animals were fed a 50:50 mix of the two diets for 14 days at 0.8x maintenance, with straw provided ad libitum. Animals were then fed one of the experimental diets for 3 weeks prior to synchronisation of oestrus and insemination and for a further 16 days following mating. All heifers were blood sampled daily from oestrus synchronisation and eight animals on each diet underwent daily transrectal real-time ultrasonography to determine the day of ovulation. All heifers were slaughtered at Day 16 after mating. While feeding of diet A (propionic) caused a significant (P<0.05) increase in the plasma insulin to glucagons ratio differences in insulin were not significantly different. This is probably due to the fact that insulin concentrations were quite high as the heifers used in the present study were in good body condition making further increases in insulin difficult to achieve. Diet did not affect size of ovulatory follicle (DIET A: 15.1+/-0.7mm; diet B: 14.6+/-0.7mm), day of ovulation (diet A: 3.5+/-0.2 days; diet B: 3.4+/-0.2 days), mean plasma progesterone concentration (diet A: 4.7+/-0.4ng/ml; diet B: 5.2+/-0.3ng/ml), corpus luteum weight (diet A: 6.0+/-0.2g; diet B: 6.0+/-0.2g) or pregnancy rate (diet A: 81.3%; diet B: 81.3%). However, the proportion of well-elongated (>10cm) embryos on Day 16 was higher in animals fed diet A than in those fed diet B (84.6% versus 38.5%; P<0.05). While progesterone concentration did not differ between pregnant and non-pregnant heifers, progesterone did show an earlier post-ovulatory rise in heifers with well-elongated (>10cm) embryos with levels in these animals significantly higher on Days 4 and 5 than in heifers with small (<10cm) embryos at slaughter. This study demonstrated an enhancement in early embryo development in animals fed a diet generating an increased insulin:glucagon ratio that was not related to circulating maternal progesterone concentrations. However, across diets, enhanced embryo development was associated with elevated plasma progesterone on Days 4 and 5 following mating.     

Effects of diets containing gossypol on ovarian histology, function and fertility in prepubertal beef heifers

Forty-five crossbred beef heifers (weight = 268.3 +/- 5.7 kg) were used to determine the effects of dietary gossypol on ovarian morphology, erythrocyte fragility and fertility. Heifers were randomly assigned to 1 of 3 isonitrogenous dietary treatments. The diet consisted of rice mill feed and milo supplemented with soybean meal (n = 13; control), cottonseed meal (n = 16; low supplementation) which supplied 6.1 g free gossypol animal(-1) day(-1), or whole cottonseed (n = 16; high supplementation) which supplied 13.7 g free gossypol animal(-1) day(-1). The heifers were group-fed each diet for 64 days and were maintained on similar but separate fescue pastures overseeded with wheat. After 64 days, 4 heifers from each diet were confined and fed their respective diets. On Day 10 following estrus, each animal was unilaterally ovariectomized, and the ovary containing the corpus luteum was removed. The remaining ovary was removed 6 to 12 hours after detection of estrus in the next cycle. Erythrocyte fragility increased (P < 0.02) in heifers receiving gossypol compared with that of the controls. Cyclicity in the heifers was 81.3, 68.8 and 38.4% for high, low and control diets, respectively, at the end of the 64-day treatment period. First service conception rate, as determined by palpation per rectum, was similar among treatments (58.3, 33.3, 33.3% for high, low and control groups, respectively). Weight gain increased (P < 0.03) in control heifers compared with that of heifers receiving gossypol. Gross ovarian morphology and histology were similar for all heifers. Although gossypol produced mild toxicosis in heifers, no adverse reproductive effects could be detected from gossypol intake.     

Effects of exogenous progesterone and cloprostenol on ovarian follicular development and first ovulation in prepubertal heifers

The objective of this study was to determine the effects of progesterone and cloprostenol (a PGF_2α analogue) on ovarian follicular development and ovulation in prepubertal heifers. In Experiment 1, crossbred Hereford heifers (Bos taurus; 10 to 12 mo old, 255 to 320 kg) were assigned randomly to three groups and given (1) an intravaginal progesterone-releasing insert (CIDR; P group, n = 13); (2) a CIDR plus 500 μg cloprostenol im (PGF_2α analogue) at CIDR removal (PPG group, n = 11); or (3) no treatment (control group, n = 14). The CIDR inserts were removed 5 d after follicular wave emergence. Progesterone-treated heifers (P and PPG groups) had a larger dominant follicle than that of the control group (P = 0.01). The percentage ovulating was highest in the PPG group (8 of 11, 73%), intermediate in the P group (4 of 13, 31%), and lowest in the control group (1 of 14, 7%; P < 0.02). In Experiment 2, 16 heifers (14 to 16 mo old, 300 to 330 kg) were designated to have follicular wave emergence synchronized with either a CIDR and 1 mg estradiol benzoate im (EP group, n = 8) on Day 0 (beginning of experiment) or by transvaginal ultrasound-guided ablation of all follicles ≥5 mm on Day 3 (FA group, n = 8). On Day 7, CIDRs were removed in the EP group, and all heifers received 500 μg cloprostenol im. Ovulation was detected in 6 of 8 heifers (75%) in both groups. In summary, the use of PGF_2α with or without exogenous progesterone treatment increased the percentage ovulating in heifers close to spontaneous puberty.     

Effects of naloxone on pethidine-induced neonatal depression. Part I--Intravenous naloxone.

Infants whose mothers had had pethidine during labour were given either naloxone 40 microgram or isotonic saline administered intravenously double-blind within one minute of birth. Peak alveolar carbon dioxide tension, carbon dioxide excretion, alveolar ventilation, feeding behaviour, and habituation to a specific sound stimulus were measured regularly up to 48 hours after birth. Alveolar carbon dioxide tension was significantly lower and alveolar ventilation significantly higher half an hour after birth in the naloxone-treated group than in the saline-treated group, but these differences between the groups were not significant at any other time, and there were no significant differences in sucking frequency or pressure, milk consumption, or habituation to the auditory stimulus.     

Effects of naloxone on pethidine-induced neonatal depression. Part II--Intramuscular naloxone.

Thirty full-term infants whose mothers had had pethidine during labour were given either naloxone 200 microgram or normal saline intramuscularly. The drugs were chosen blindly and administered within one minute of birth. Naloxone produced a significant reduction in mean alveolar carbon dioxide tension and an increase in carbon dioxide excretion and mean alveolar ventilation at all times up to 48 hours after birth. The mean rate of habituation to a repeated auditory stimulus, the mean sucking frequency, the sucking pressure, and the mean consumption of milk were all significantly higher in the naloxone-treated group than in the placebo-treated group up to 48 hours after birth. Intramuscular naxolone therefore seemed to reverse the undesirable effects of pethidine.     

The effect of photoperiod on serum concentrations of luteinizing and follicle stimulating hormones in prepubertal heifers following ovariectomy and estradiol injection

Eleven heifers, between 63 and 197 days of age, were exposed to 18 hr light/day (L) or natural photoperiods (N), beginning October 19, 1979. They were ovariectomized 8 weeks later. LH concentrations after ovariectomy were not affected by photoperiod, but the rate of increase of FSH after ovariectomy was greater (P<0.10) for group L than for group N. Three weeks after ovariectomy, heiters were injected, IV, with 0.1 mug/kg estradiol-17beta. LH concentrations initially decreased after injection. This was followed by a series of pulses larger than those prior to injection. FSH concentrations declined after injection and remained low throughout the sampling period. The net response of LH concentrations to estradiol (mean post-injection concentration minus mean pre-injection concentration) was greater (P=0.05) for group L (4.7 +/- 0.49 ng/ml) than for group N (2.9 +/- 0.37 ng/ml). Photoperiod did not affect the net response of FSH concentrations to estradiol. We concluded that exposing prepubertal heifers to 18 hr light/day during the winter resulted in a greater rate of increase of FSH after ovariectomy and greater estrogen-induced LH release. Because the response of LH to estradiol-17beta differed from the response of FSH, these hormones may be regulated differently.     

Morphine, naloxone and the gonadotrophin surge in ewes

Possible endogenous opioid peptide regulation of the preovulatory gonadotrophin surge was examined in ewes during the breeding season. Intact ewes (n = 54) were synchronized by treatment for 12 days with intravaginal sponges releasing medroxyprogesterone acetate. Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion prior to and during the gonadotrophin surge were not affected by naloxone (0.33 mg/kg body wt per h) administered from the time of medroxyprogesterone acetate withdrawal until 30 h after the onset of oestrus (n = 6). Morphine was administered in 4 patterns: (i) 0.25 mg morphine/kg body wt per h from medroxy-progesterone acetate withdrawal until 30 h after the onset of oestrus (n = 6), (ii) 0.25 mg morphine/kg body wt per h from 24 to 48 h after medroxyprogesterone acetate withdrawal (n = 6), (iii) 0.50 mg morphine/kg body wt per h from 24 to 36 h after medroxyprogesterone acetate withdrawal (n = 6) and (iv) 0.50 mg morphine/kg body wt per h from 18 to 30 h after medroxyprogesterone acetate withdrawal (n = 6). Oestrus and the gonadotrophin surge were delayed, but not blocked, in all cases of morphine administration (P less than 0.05). Inconsistent effects of morphine on circulating oestradiol and gonadotrophin concentrations prior to the gonadotrophin surge suggest that the delays are not due to reduced gonadotrophic support of ovarian oestradiol output. Morphine may reduce responsiveness of central behavioural and gonadotrophin surge-generating centres to the oestradiol signal. The absence of effects of naloxone on gonadotrophin secretion suggest that suppression of LH secretion by opioid peptide activity is reduced after the end of the luteal phase.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of bovine follicular fluid inhibin on serum gonadotrophin concentrations in ewes during oestrus

Experiments were conducted with ewes to investigate the effects of an enriched bovine follicular fluid inhibin preparation (INH) on gonadotrophin secretion after the onset of oestrus. Administration of INH (10 mg) 1 h after the onset of oestrus did not significantly alter the preovulatory FSH and LH surges or the second FSH peak. To determine the effects of INH on the second FSH surge, ewes were treated with saline (N = 7) or INH (N = 10) at 4 h (10 mg) and 24 h (5 mg) after the peak of the preovulatory LH surge. The second FSH surge was delayed about 24 h (P less than 0.05) in ewes treated with INH; however, the delay did not alter the interval to the next oestrus. In a third experiment, 16 ewes were assigned to 4 groups in a 2 x 2 factorial with the main effects being ovariectomy at 4 h and INH treatment (10 mg) at 4, 20 and 36 h after the peak of the LH surge. Controls received sham ovariectomy and saline injection as appropriate. Ovariectomy resulted in a rapid increase in serum FSH but not LH and this was delayed (P less than 0.05) by INH treatment. These results indicate that inhibin has a selective inhibitory action on FSH secretion in ewes and suggests that the second FSH surge results from increased basal FSH secretion due to decreased endogenous inhibin levels.     

Jugular plasma concentrations of 13,14-dihydro-15-keto-prostaglandin F2 alpha in prepubertal beef heifers treated with progestogen then challenged with oxytocin.

Prepubertal Angus crossbred heifers (n = 24) between 8 and 10 mo of age were used to determine if progestogen treatment would enhance jugular concentrations of 13,14-dihydro-15-keto-prostaglandin F2 alpha (PGFM) after oxytocin (OT) injections. Heifers were stratified by age and weight and allotted to randomized treatments in a 2 x 2 factorial arrangement. Heifers were treated with either a norgestomet (NOR) implant (6 mg) for 9 d or no implant (0 mg; BLK). On d 8 of NOR treatment, jugular veins were catheterized and, on d 9, blood samples were collected every 15 min for 165 min. The first four samples were used to determine basal PGFM concentrations (an indirect measure of uterine PGF2 alpha release). After collection of the fourth sample, either OT (100 IU) or saline (0 IU; SAL) was injected via the jugular catheter. After the 165-min sample was collected, NOR implants were removed. Beginning 48 h after implant removal, a second 165- min blood sampling period was initiated. Average progesterone concentrations were less than 1 ng/ml during both bleeding periods. Within treatment, PGFM concentrations were similar between the first and second sampling periods; therefore, data within treatment were combined. Basal PGFM concentrations were higher (P < .01) in NOR-treated than in BLK heifers. Oxytocin did not increase PGFM concentrations in BLK-OT heifers; however, a marked increase in PGFM was detected in the NOR-OT heifers in response to oxytocin. Average PGFM concentration was greatest (P < .0001) in NOR-OT heifers, and PGFM profiles differed (P < .0001) between NOR-OT and each of the other treatment groups. Results from this study indicate that NOR increases basal PGFM and may "condition" the uterus to respond to OT in prepubertal heifers.     

Ovarian response of prepubertal Murrah heifers to exogenous GnRH

The aim of the current study was to evaluate the ovarian follicular response to GnRH at two dose levels in prepubertal Murrah buffalo heifers during hot-humid months. Heifers received 10μg (Group A, n=7) or 20μg (Group B, n=6) of GnRH during the growing phase of the dominant follicle (DF>6mm), as identified by pretreatment regular ultrasound scanning. A similar dose was repeated approximately 30 days later, again coinciding with another growing DF. Post-treatment ovarian activity was studied by alternate day transrectal ultrasonography until 25 days post-second GnRH. In both groups, three types of response were recorded: formation of a corpus luteum (CL), luteinization of the follicle wall, i.e. luteinized follicle (LF) or no response (NR). Pooled data at GnRH injections revealed that the DF diameter in relation to the type of response in Group A heifers was, NR: 6.83±0.17 (n=3), LF: 8.10±0.67 (n=6) and CL: 8.88±0.96mm (n=5), while respective values in Group B heifers were 8.68±0.44mm (n=4), 9.37±0.46 (n=6) and 8.95±0.55mm (n=2). The pattern of response was more evident in Group A but unpredictable in Group B. Corpora lutea developed in response to both the GnRH injections were short-lived. Mean maximum diameter of LFs in Groups A and B were 16.07±0.99 and 16.58±1.20mm respectively and wave duration of these follicles in both groups was 23.33±1.67 and 25.33±1.12 days, respectively. None of the heifers commenced cyclicity following regression of the CL or LF.     

Effects of season of birth on the prepubertal pattern of gonadotropin secretion and age at puberty in beef heifers

There is an early transient rise in gonadotropin secretion in spring-born prepubertal heifers and there is an indication that this pattern is different in autumn-born heifers. The effect of season of birth on age and weight at puberty is equivocal. This study was designed to compare the temporal patterns of LH and FSH secretion between spring- and autumn-born heifers and to determine the effects of season of birth on age and weight at puberty. Blood samples from 2 groups of heifer calves born in spring (last week of March, n = 5) or autumn (last week of October, n = 5) were collected every other week from birth to puberty and every 15 min for 10 h at 6, 12, 18, 24 and 32 wk of age. Timing of puberty was determined by measuring progesterone in plasma samples collected every 2 to 3 d starting at 42 wk of age. Age and weight at onset of puberty did not differ between the 2 groups of heifers (P > 0.05); however, the autumn-born heifers tended to mature in a wider range of ages and weights. Based on the 10-h sampling periods, mean serum concentrations of LH and LH pulse frequency and amplitude were higher in spring-born heifers at 18 wk of age than in autumn-born heifers (P < 0.05). In spring-born heifers, LH pulse frequency increased over time to 32 wk of age, and LH pulse amplitude was higher at 12 and 18 wk than at 32 wk of age (P < 0.05). Autumn-born heifers had higher LH pulse frequency at 6 wk and showed a decrease in mean concentrations of LH at 12 and 18 wk of age (P < 0.05). The FSH pulse frequency of spring-born heifers was higher at 12 wk of age than in autumn-born heifers (P < 0.05), FSH pulse amplitude in autumn-born heifers decreased from 6 to 32 wk of age. It was concluded that although the mean age and weight at puberty did not differ between spring- and autumn-born heifers, the range in age and weight at puberty was wider in the autumn-born heifers. The patterns of LH secretion differed between spring- and autumn-born prepubertal heifers, with spring-born calves exhibiting an early rise in LH secretion, while mean serum concentrations of LH decreased during this period in autumn-born heifers.     

Raised serum human chorionic gonadotrophin concentrations in hyperemesis gravidarum.

Serum human chorionic gonadotrophin (HCG) concentrations were determined by radioimmunoassay with an antiserum specific to HCG beta-subunit in 42 patients with hyperemesis gravidarum and 115 women with normal pregnancies. Mean concentrations (+/- SE of mean) were higher in the women with hyperemesis gravidarum at 7-8 weeks (40.8 +/- 5.2 IU/ml v 22.1 +/- 1.4 IU/ml; P less than 0.001), 9-11 weeks (38.1 +/- 2.3 IU/ml v 27.1 +/- 2.1 IU/ml; P less than 0.0025), and 12-14 weeks of gestation (35.9 +/- 4.2 IU/ml v 25.1 +/- 1.7 IU/ml; P less than 0.005), but there was no difference between the two groups at 15-20 weeks of gestation. In the hyperemesis gravidarum group primigravid women had a higher (P less than 0.005) mean HCG concentration (41.8 +/- 4.0 IU/ml) than multigravid women (32.2 +/- 2.3 IU/ml). The results suggest a causal relation between a high serum HCG concentration and hyperemesis gravidarum.     

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.     

Effects of an agonist of gonadotrophin releasing hormone in cattle. I. Hormone concentrations and oestrous cycle length

Four trials were completed to study the effects of a single intramuscular injection of 5 μg of an agonist of gonadotrophin releasing hormone (Hoe 766) on plasma concentrations of LH and progesterone, and on oestrous cycle length in normally cycling dairy cows.The first trial (four cows) showed that a mid-cycle injection of Hoe 766 temporarily increases plasma LH from less than 5 ng/ml to over 20 ng/ml within 2.5 h. Average plasma progesterone concentrations ranged from 4.8 to 7.0 ng/ml compared to 3.3 ng/ml in the control animal.The second trial (22 cows) showed that an injection of Hoe 766 on Cycle Day 3, 6 or 9 (Oestrus = Cycle Day 0) increased average plasma progesterone concentrations during Cycle Days 13, 14 and 15 by 1.2 ng/ml. Each of three cows injected on Cycle Day 16 maintained plasma concentrations above 3.9 ng/ml until Cycle Day 19 and corpus luteum (CL) size was maintained until Cycle Day 21. Except for the group of cows injected on Cycle Day 3, all other groups had temporarily reduced concentrations of plasma progesterone when sampled 24 h after Hoe 766 administration.The third trial (216 cows) showed that a single injection of Hoe 766 made between Cycle Day 1 and 10 did not alter oestrous cycle length (21.5 vs 21.3 days). In contrast, in the fourth trial (371 cows), a single injection of Hoe 766 between Cycle Days 12 to 16 altered the distribution of cycle lengths of 17–29 days, the average cycle length and the incidence of ovulation without detected oestrus. Compared to matched control cows, fewer Hoe 766-treated cows were detected in oestrus (73.9% vs 90%), or had cycle lengths of less than 20 days (4.7% vs 22.2%). These effects were most pronounced among cows injected on Cycle Day 16 when only 51.7% were detected in oestrus and their average cycle length was 24.1 days.These effects were not due to the formation of a secondary CL. Rather, the injection of Hoe 766 stimulated CL function and appeared to prevent or delay normal luteolysis when administered from Cycle Day 12.     

Effects of an LHRH antagonist on gonadotrophin and oestradiol secretion, follicular development, oestrus and ovulation in Holstein heifers

Mature cyclic Holstein heifers were given a luteolytic dose of cloprostenol followed by two i.v. injections, 12 h apart, of various doses of [Ac-D-Nal1, D-p-Cl-Phe2, D-Trp3, D-Arg6, D-Ala10]-LHRH, beginning either at the time of first observation of behavioural oestrus, or 48 h after the cloprostenol injection. When treatment began at the first observation of oestrus, the time of ovulation, as determined by ultrasonic echography, was significantly delayed by total doses of 0.8 mg or more of the antagonist. When given at 48 and 60 h after cloprostenol injection, a total dose of 1.5 mg of the antagonist significantly delayed the growth of the ovulating follicle, the onset of oestrus, the preovulatory surges of oestradiol, LH and FSH, and ovulation. It is concluded that the LHRH antagonist can effectively suppress endogenous LH secretion and may therefore be useful in the study of follicular development, ovulation, and other events in the oestrous cycle of the cow.