Peripheral Progesterone Concentrations Research Articles

Involvement of β-adrenoceptors in the regulation of luteal function in cattle

The infusion of β-adrenomimetic drug (bamethan sulphate—BS) on day 11 of the oestrous cycle markedly increased peripheral progesterone concentration whereas infusion of β-antagonist (propranolol) decreased plasma progesterone levels in heifers. Oxytocin injected on days 2–6 of the oestrous cycle shortened the length of the cycle (to 7–9 days) whereas the length of the cycle in heifers injected with oxytocin followed by injections of either BS or xanthinol (theophylline nicotinate—a potent vasodilatory drug) was normal (21.8 ± 1.7). Heifers injected with BS on days 13–21 of the oestrous cycle had increased levels of progesterone on days 13–19 of the cycle and the length of the cycle was normal (22.3 ± 0.6 compared with 21.3 ± 1.5 for the control). The data indicate that: 1. (1) β-adrenergic stimulation overcomes the action of oxytocin given on days 2–6 of the cycle, whereas the stimulation of β -receptors on days 13–21 of the oestrous cycle raised progesterone plasma concentration but did not prevent spontaneous luteolysis. 2. (2) Basal secretion of progesterone in cattle is partly dependent on constant β-adrenergic stimulation.

Increased ovulation rate in androstenedione-immune ewes is not due to elevated plasma concentrations of FSH

Two experiments were undertaken to determine the hormonal response of Merino ewes to immunization against androstenedione (Fecundin). In Exp. 1 peripheral concentrations of LH, FSH and progesterone were monitored in spontaneously cycling ewes (20 immunized and 21 controls). In Exp. 2 (10 immunized and 10 controls) the same hormones were measured in ewes before and after prostaglandin (PG)-induced luteolysis and, in addition, the pattern of pulsatile LH secretion was determined during the luteal (PG + 12 days), early follicular (PG + 24 h) and late follicular (PG + 40 h) phase of the oestrous cycle. Ovulation rates were measured in both experiments. The results of these experiments indicate that androstenedione-immune animals have elevated ovulation rates (0.6-0.7 greater than control animals; P less than 0.05) associated with elevated plasma concentrations of LH and progesterone. The magnitude of the increase in plasma progesterone was correlated with androstenedione antibody titre (r = 0.6, P less than 0.001). LH pulse frequency of androstenedione-immune ewes tended to be higher at all stages of the oestrous cycle, but this difference was only significant (P less than 0.05) during the luteal phase. Mean plasma concentrations of FSH did not differ significantly between immunized and control ewes at any stage of the cycle. Analysis of periodic fluctuations in FSH during the luteal phase revealed that androstenedione-immune animals had a similar number of fluctuations of a similar amplitude to those of control animals, but the nadir of these fluctuations was lower (P less than 0.05) in immunized animals. A significant (P less than 0.05) negative correlation existed between androstenedione antibody titre and the interval between FSH peaks (r = -0.49) and androstenedione antibody titre and FSH nadir concentrations (r = -0.46). It is concluded that plasma FSH concentrations are not a determinant of ovulation rate in androstenedione-immune ewes and that increased LH concentrations, or perturbation of normal intraovarian mechanisms, may be responsible for the increase in ovulation rate observed in ewes immunized against androstenedione.

Effect of relaxin on facilitation of parturition in dairy heifers

Purified pig relaxin (3000 U/mg) was injected i.m. into pregnant Holstein dairy heifers on Day 276 or 277 to determine its effect on parturition and sequential measurements of the pelvic area, cervical dilatation, and peripheral blood-plasma concentrations of progesterone and relaxin. Treatments included phosphate-buffer saline (2 ml, Group C, N = 7), relaxin once (1 mg, Group 1R, N = 7), and twice (2 mg, 12 h apart; Group 2R, N = 7). Intervals (mean +/- s.e.) between the first injection of relaxin or PBS and calving were 64 +/- 17, 80 +/- 19 and 125 +/- 34 h for Groups 2R, 1R and C, respectively. The calving intervals were reduced in Groups 2R (P less than 0.01) and 1R (P less than 0.05) compared with Group C. The incidence of dystocia was 29% (2 of 7) in Group 2R and 43% (3 of 7) in Group 1R compared with 57% (4 of 7) in Group C (P less than 0.01). Body weights and ratios of males to females of the calves were similar (P greater than 0.05) between groups. Progesterone plasma concentrations decreased (P less than 0.01) earlier in Groups 1R and 2R compared with Group C, and this acute decrease began within 6 h of treatment. At 24 h after relaxin or PBS injection, progesterone concentrations were 2.7 +/- 1.1 ng/ml for Group 2R, 3.5 +/- 0.9 ng/ml for Group 1R, and 6.0 +/- 0.1 ng/ml for Group C. Relaxin reached peak blood-plasma levels of 19 +/- 2.2 ng/ml 1 h after injection of relaxin, but remained unchanged, 0.3 +/- 0.01 ng/ml, in Group C. Pelvic area was increased 26%, 22% and 14% and cervical dilatation was increased 109%, 76% and 53% 48 h after injection in Groups 2R, 1R and C, respectively, but these responses were similar among groups at the time of parturition. We conclude that two i.m. injections of relaxin facilitated earlier calving, acutely decreased progesterone secretion, increased cervical dilatation and pelvic area expansion, and decreased the incidence of dystocia in dairy heifers.

Effect of Aspiration of the Preovulatory Follicle on Luteinization, Corpus Luteum Function, and Peripheral Plasma Gonadotropin Concentrations in the Mare

Follicular fluid from small- to medium-sized follicles has been shown to have an inhibiting effect on luteinization of granulosa cells in vitro. This study was conducted to investigate the effect of in vivo removal of follicular fluid on luteinization, peripheral gonadotropin concentrations, and ovulation of secondary follicles in the mare. Follicular fluid was aspirated from the preovulatory follicles of mares when the diameter of the follicle was 30-34 mm (Group A), 35-39 mm (Group B), or 40-44 mm (Group C). Mares in Group D served as controls and the preovulatory follicle was not aspirated. Mares in Group A had a significantly earlier rise in peripheral progesterone concentrations than did controls. There was no difference in duration of progesterone secretion or peak progesterone production between groups. LH and FSH values were significantly higher for mares in Groups A and B than for control mares. Mares in Group A tended to have a higher incidence of secondary ovulations than did mares in other groups. These data support the in vitro findings that follicular fluid from small- to medium-sized follicles may contain a luteinization inhibitor, and indicate that presence of follicular fluid during the final days of follicular maturation is not essential for development of a normal CL.

Autoradiographical localization of oxytocin binding sites on ovine oviduct and uterus throughout the oestrous cycle.

A highly specific oxytocin receptor ligand, 125I-labelled d(CH2)5[Tyr(Me)2,Thr4,Tyr-NH9(2)] vasotocin (125I-OTA), was used to localize high affinity oxytocin receptors in ovine uterine and oviduct tissues throughout the oestrous cycle. The pattern of binding revealed by in vitro autoradiography correlated well with the results of the homogenate receptor assays using the same ligand and with previous binding assays using the tritiated ligand. At oestrus, specific 125I-OTA binding was evident on the luminal epithelium of the caruncular and intercaruncular regions, on the epithelial cells lining the secretory uterine glands and in the stroma underlying the caruncular epithelium. In the myometrium diffuse labelling was evident in the outer longitudinal smooth muscle layer. At Day 4 of the cycle, binding to the stroma was diffuse and virtually absent from the glandular epithelium. No specific binding was evident in either tissue at Day 12 of the luteal phase, but by Day 14, prior to the decrease in peripheral progesterone concentrations, binding was again apparent on the luminal epithelium only. Specific binding to the oviduct was localized to the smooth muscle layer of the isthmus region of oestrous ewes and was not detected at any other stage of the oestrous cycle. These studies extend our knowledge of the distribution of oxytocin binding sites in uterine and oviduct tissues throughout the oestrous cycle and suggest that oxytocin has an important role in stimulating oviduct and uterine motility at a time crucial to successful egg collection and/or sperm embryo transport.

Peripheral concentrations of immunoreactive inhibin during pregnancy and parturition in the ewe

The aim of this study was to measure the peripheral concentrations of immunoreactive inhibin (ir-inhibin) and progesterone (P) during pregnancy and parturition in the ewe and to relate the concentrations of ir-inhibin to P and to the number and sex of the fetuses. P increased across pregnancy with higher levels in ewes with 2 fetuses (n = 5) than in those with 1 fetus (n = 6), and concentrations falling before birth. ir-Inhibin concentrations were relatively stable during the first 40 days of pregnancy for example (Day 20, 34.7 +/- 2.9 pmol L-1; mean +/- s.e.m., n = 11). After Day 40, inhibin fell in all ewes to reach less than or equal to 2.5 pmol L-1 after Day 80 (mean on Day 103, 6.3 +/- 1.2 pmol L-1), and remained low until 2 days before parturition when concentrations rose sharply, peaking at or around the day of birth in all ewes (21.5 +/- 2.1 pmol L-1). Thereafter, ir-inhibin fell and remained low or undetectable for up to 10 days in the six ewes still being sampled. ir-Inhibin concentrations in ewes carrying one (n = 6), two (n = 5) or three fetuses (n = 1) did not differ at any stage of pregnancy examined. The sex of the fetus did not appear to influence the peripheral concentrations of ir-inhibin in the ewe.

Exfoliative vaginal cytology during the oestrous cycle of the cow, after ovariectomy, and after exogenous progesterone and oestradiol-17β

Vaginal exfoliative cytology was studied in five normal cyclical Friesian cows over 10 oestrous cycles each. Oestrus was assessed by the use of KaMaR heat mount detectors, whilst progesterone concentrations were measured in the peripheral blood using radioimmunoassay. Apart from oestrus, the exfoliative cytology was not consistent in appearance and did not appear to be influenced directly by the changes in endogenous ovarian steroid hormones. Four of the cows were subsequently ovariectomized and the vaginal exfoliative cytology monitored before and after the administration of 2 g of progesterone in a slow release vehicle and, following a rest period, after the administration of 12·5 mg oestradiol benzoate in oil. Peripheral progesterone and oestradiol-17β concentrations were measured in the peripheral plasma. Although the total number of epithelial cells increased after both hormone treatments no specific changes were observed.

Luteal inadequacy during the early luteal phase of subfertile cows

A study was made of early luteal function (up to Day 6) in cyclic and pregnant heifers and also in older, subfertile cows. There were no differences in vivo or in vitro between cyclic and pregnant heifers, indicating no luteotrophic effect of the embryo at this stage, but the increase in postovulatory peripheral progesterone concentrations was delayed (P less than 0.01) and occurred more slowly (P less than 0.001) in the subfertile cows than in the heifers. The corpora lutea of the subfertile cows were heavier (P less than 0.001) than those of the heifers on Day 6. Basal progesterone production by dispersed luteal cells was similar between heifers and subfertile cows, but there was a difference (P less than 0.001) in the pattern of response to exogenous LH and PGE-2. Cells from subfertile cows were less sensitive to the stimulatory effects of PGE-2 and although LH increased (P less than 0.001) progesterone production by all cells, this stimulation by a low dose of LH was inhibited by PGE-2 in luteal cells from subfertile cows. This effect did not occur in the luteal cells from heifers. These results indicate the possibility that luteal inadequacy, due to a diminished response to circulating luteotrophic hormones, may contribute to embryo mortality in subfertile cows.

Peripheral and Ovarian Venous Concentrations of Estradiol and Progesterone in Postmenopausal Women with “Non‐Endocrine” Ovarian Tumors

Estradiol (E2) and progesterone (P4) levels in peripheral venous blood were measured in 45 postmenopausal (more than 5 years after menopause) women with ovarian neoplasms. The E2 and P4 concentrations in ovarian venous blood were also measured in 26 of these 45 patients. The steroid levels were compared with the values in 10 postmenopausal women without ovarian tumors. In peripheral venous blood, relatively high E2 concentrations were found in 24 of the 45 (53%) postmenopausal patients with ovarian tumors, and relatively high P4 concentrations were noted 15 of 45 (33%) of the patients. In ovarian venous blood, relatively high values of E2 were found in 18 of 26 (69%) of the postmenopausal patients, and relatively high values of P4 were noted in 11 of these 26 (42%). No differences in the E2 and P4 levels in peripheral and ovarian venous blood were noted between the patients with benign tumors and those with malignant tumors. These results suggest that some of the epithelial ovarian tumors in postmenopausal women have steroid-producing activity.

Pregnancy, lactation and the oestrous cycle of the pouched mouse, Saccostomus campestris.

The anatomy and histology of pouched mouse ovaries were studied during the oestrous cycle, pregnancy and lactation along with the relationship between the ovarian structures and circulating concentrations of progesterone. The structure of the ovaries resembled that of most rodents. Follicular development indicated that ovulation takes place on the night between pro-oestrus and oestrus, i.e. at the time when mating normally occurs. Corpora lutea were accumulating in cyclic females, while successively disappearing during pregnancy, leaving only the set formed after conception. After parturition luteal regression was rapid. Theca interna, included in the corpora lutea, formed glandular stromal tissue after regression of the luteal tissue formed from granulosa cells. The progesterone profile of non-pregnant females indicated a short but functional luteal phase (peak at metoestrus) during the cycle. During pregnancy three peaks of progesterone stood out: (1) when implantation starts, (2) when older sets of corpora lutea showed rejuvenation and placental signs were found in the vaginal smears, and (3) 3 days before expected parturition when luteal development (as judged by histology) reached a peak. The placenta may participate in but not 'take over' the progesterone production during later stages of pregnancy. Very low concentrations of peripheral progesterone during lactation and a very low level of follicular development at that time support an earlier suggestion of a lactational anoestrus in pouched mice.

Uterine activity in cows during the oestrous cycle, after ovariectomy and following exogenous oestradiol and progesterone

Uterine activity was monitored in three, 2-year-old nulliparous Ayrshire heifers using intrauterine balloon-tipped catheters and pressure transducers during the oestrous cycle, after ovariectomy and following the intravenous infusion of progesterone and oestradiol-17β. During the oestrous cycle uterine activity, as measured by the frequency and amplitude of contractions, was greatest around oestrus and declined during the luteal phase of the cycle; there was a close correlation with peripheral progesterone concentrations. In two animals after bilateral ovariectomy spontaneous uterine activity persisted, whilst in the third animal the uterus was quiescent. In the first two heifers intravenous progesterone infusions reduced the spontaneous uterine activity, eventually completely abolishing it. There was evidence of a dose response effect at the two infusion rates. Oestradiol benzoate infusions initially inhibited spontaneous uterine activity before stimulating contractions with some evidence of a dose relationship. As demonstrated in normal cyclical and steroid-infused animals, uterine activity appears to be under the influence of both hormones although the influence of progesterone is greater.

Studies on increasing breeding frequency in the ewe. 2. The endocrine status of lactating ewes induced to ovulate 28, 35 or 42 days post-partum

Forty-two Finn Dorset ewes each suckling two lambs were divided into three groups ( n=14/group), induced to ovulate and intra-uterine inseminated at 28, 35 or 42 days post-partum. To assess the activity of the hypothalamic-pituitary-ovarian axis during the early post-partum period and prior to oestrous induction, blood samples were taken from all ewes at 15 minute intervals for 12 h on Day 14 post-partum, from the 35- and 42-day groups on Day 21 and the 42-day group on Day 28. No change in plasma concentrations of FSH or the pulsatile secretion of LH were observed with time after parturition, despite high prolactin levels, suggesting that inadequate gonadotrophin priming was not limiting the induction of ovulation in the early post-partum ewe. Of the 14, 12 and 11 ewes exhibiting behavioural oestrus, pre-ovulatory LH/FSH surges were detected in 11, 10 and 9 ewes. For these ewes the timing of the surges relative to oestrous onset and intra-uterine insemination time did not change with time post-partum. The magnitude of the pre-ovulatory LH and FSH surges was significantly lower ( P<0.05) in ewes induced to ovulate 42 days post-partum. No correlation existed between the timing and magnitude of the pre-ovulatory surges and subsequent fertilisation rate, luteal function or pregnancy rate. These results suggest that gonadotrophin secretion prior to and during an induced peri-ovulatory phase in lactating ewes was not limiting the establishment of pregnancy in the early post-partum. The incidence of inadequate luteal function as assessed by twice-daily determinations of peripheral progesterone concentrations post-oestrus was high in ewes induced to ovulate 28 days post-partum and was associated with low recovery rates of unfertilised ova. In contrast luteal inadequacy was not limiting the establishment of pregnancy in ewes with fertilised eggs recovered in the 35- and 42-day groups.

Effect of melatonin on the peripheral concentrations of LH and progesterone after oestrus, and on conception rate in ewes.

It has previously been shown that administration of the indoleamine melatonin to advance the breeding season of ewes is also associated with an increase in ovulation rate and subsequent litter size. Experiment 1 assessed whether, in ewes receiving melatonin to advance the breeding season, the indoleamine acts directly on the corpus luteum to enhance progesterone secretion or indirectly through increased activity of the hypothalamic pulse generator. Six ewes received 3 mg melatonin orally at 15.00 h daily from 22 March onwards, six were induced to ovulate during mid-anoestrus following withdrawal of a progestagen pessary and injection of exogenous gonadotrophin and six acted as naturally ovulating controls. First overt oestrus occurred between 17 May and 8 July in melatonin-treated ewes, between 21 October and 3 January in control ewes and on 8 July in all induced ewes. On days 2 and 10 after the first overt oestrus, melatonin-treated ewes had pulsatile LH activity characteristic of that measured in control ewes ovulating naturally during the breeding season. There was an absence of any pulsatile LH activity in the induced ewes. Progesterone concentrations between days 7 and 12 following oestrus were significantly higher in melatonin-treated than in control and induced ewes, suggesting a luteotrophic role for melatonin. Experiment 2 was carried out to determine whether administration of melatonin commencing after induced ovulation and insemination would alter the endocrine status of the ewe and thereby influence the establishment of pregnancy and embryo survival.(ABSTRACT TRUNCATED AT 250 WORDS)

In-vitro and in-vivo responsiveness of the corpus luteum of the mare to gonadotrophin stimulation.

Dispersed horse luteal cells were used to evaluate the ability of horse LH, hCG and PMSG to stimulate progesterone secretion in vitro. Morphological characterization of these cells before gonadotrophin stimulation indicated the presence of two populations of cells based on cell diameters. In luteal cells incubated as suspended cells, horse LH and hCG stimulated (P less than or equal to 0.05) progesterone production at all levels of treatment. Stimulation of progesterone secretion by hCG was greater (P less than or equal to 0.05) than by horse LH over the range of concentrations utilized. When mares (N = 7) received an intramuscular injection of 1000 i.u. hCG on Days 3, 4 and 5 after the end of oestrus, there was an increase (P less than or equal to 0.05), in peripheral progesterone concentrations beginning on Day 7 and continuing until Day 14 compared with controls (N = 7). Peripheral progesterone concentrations continued to be elevated in hCG-treated mares for Days 15-30 after oestrus in those mares that conceived. Although treatment with hCG increased progesterone concentrations, it had no influence on anterior pituitary release of LH as measured by frequency and amplitude of LH discharge. We conclude that the mare corpus luteum is responsive to gonadotrophins in vitro and that exogenous hCG can enhance serum progesterone concentrations throughout the oestrous cycle and early pregnancy.

Reproduction in female Hartmann's zebra, Equus zebra hartmannae.

Ovaries, fetuses and plasma were collected from zebra mares shot in the Etosha National Park in Namibia between 15 and 25 August 1983. Ovarian weight was affected by reproductive status and most of the non-pregnant mares were anoestrous. The number of follicles varied between individuals and only pro-oestrous/oestrous mares had follicles larger than 20 mm in diameter. The largest follicle in pregnant mares was only 9 mm in diameter. Corpora lutea and corpora albicantia were found in non-pregnant as well as pregnant mares: 4 pregnant mares had only corpora albicantia. The presence of secondary corpora lutea could not be confirmed in any of the pregnant mares. Implantation was estimated to occur at around 73 days of gestation, and most mares (84%) had conceived between November and April. Peripheral concentrations of plasma progesterone during pregnancy varied from 0.5 to 2.4 ng/ml.

A possible role for testosterone during luteolysis in the ewe

Increased peripheral plasma concentrations of testosterone were detected at the commencement of luteolysis and at oestrus in sheep. Daily subcutaneous administration of spironolactone (10 mg kg-1) between days 10 and 16 of the oestrous cycle suppressed testosterone secretion and delayed luteolysis and oestrus. In pregnant ewes no increases in testosterone concentrations occurred between days 10 to 16 after mating and luteal regression, monitored by peripheral progesterone concentrations, was inhibited. It is suggested that aromatisation of testosterone to oestrogens is needed for luteolysis in sheep.

Preovulatory evaluation of the superovulatory response in donor cattle

Dairy cows and heifers (n = 134) were induced to superovulate with exogenous gonadotrophins. In 103 animals, peripheral plasma concentrations of progesterone (P4) and luteinizing hormone (LH) were measured during the preovulatory period. On the basis of these measurements, normal and deviating profiles of P4 and LH were defined. A high degree of correlation existed between the normality of the two profiles; when the P4 profile was normal, the probability for the LH profile also to be normal was greater than 10:1. This relationship was utilized to evaluate donors based on four preovulatory measurements of P4. When used on 31 animals used for collection of eggs, a superior superovulatory response was encountered in animals with normal vs deviating P4 profiles (eggs recovered: 7.2 ± 1.1 vs 0.5 ± 0.3, P < 0.001; transferable embryos: 4.4 ± 0.9 vs 0.3 ± 0.2, P < 0.01). It is concluded that evaluation of donors by measurements of progesterone in plasma at four preovulatory sampling points allows for the early exclusion of donors with inferior embryo yield.

Function of abnormal corpora lutea in vivo after GnRH-induced ovulation in the anoestrous ewe.

Anoestrous Romney Marsh ewes with and without progesterone treatment (+P, -P) were treated with small-dose (250 ng) multiple injections of GnRH at 2-h intervals for 48 h. Animals were slaughtered on Days 4, 5, 7 and 11 after the end of GnRH treatment and luteal function was assessed by the measurement of daily plasma progesterone concentrations. In all animals which ovulated (29/32, 91%) peripheral progesterone concentrations rose to 0.5-1.0 ng/ml within 3 days of the end of GnRH treatment. In 7/7 (100%) +P animals and 5/22 (23%) -P animals, progesterone concentrations continued to rise and were maintained at levels greater than 1.5 ng/ml until slaughter. In the remaining -P animals, plasma progesterone concentrations declined to reach basal levels by Day 5. Corpora lutea recovered from these animals showed signs of premature regression on Day 5 and were fully regressed by Day 7. Progesterone priming delayed the occurrence of the LH surge which occurred 39.1 +/- 3.6 h after the end of GnRH treatment in the +P animals compared to 20.2 +/- 1.74 h (P less than 0.001) in the -P animals in which luteal function was abnormal and 22.4 +/- 4.35 h in the -P animals in which luteal function was normal. These results show that abnormal luteal function occurs in the majority of GnRH-treated ewes in the absence of progesterone pretreatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of exogenous progesterone on prostaglandin F 2α release and the interestrous interval in the bovine

The present study was developed to determine if administration of progesterone, early in the estrous cycle of the cow, stimulated an advanced pulsatile release of PGF 2α from the uterine endometrium resulting in a decreased interestrous interval. Twenty-three cyclic beef cows were randomly assigned to receive either sesame oil or progesterone (100 mg) on Day 1, 2, 3 and 4 of the estrous cycle. Peripheral plasma concentrations of progesterone and the metabolite of prostaglandin F 2α, 15-keto-13, 14-dihydro-prostaglandin F 2α (PGFM) were measured by radioimmunoassay. Administration of exogenous progesterone increased peripheral plasma concentration of progesterone in treated (3.67 ng/ml) compared to control (1.28 ng/ml) cows from Day 2 through 5 of the estrous cycle. Progesterone administration shortened the interestrous interval (16.7 d) compared to controls (21.6 d). The shortened interestrous intervals in treated cows resulted from an earlier decline in peripheral plasma progesterone. Decline of peripheral plasma progesterone concentrations is coincident with an increased pulsatile release of PGFM in both progesterone treated and control cows. Results indicate that administration of exogenous progesterone stimulates an earlier maturation of endometrial development, causing an advanced release of PGF 2α which shortens the interestrous interval of the cow.

Plasma concentrations of progesterone, oestrogens, testosterone and LH-like activity during the oestrous cycle of the camel (Camelus dromedarius)

Peripheral plasma concentrations of progesterone, total oestrogens and testosterone (measured by RIAs) and LH (monitored by the mouse Leydig cell bio-assay) were measured in 8 female camels for a complete oestrous cycle (23.1 +/- 1.2 days). The absence of an LH surge and a low concentration of progesterone (less than 1 ng/ml) during oestrus (5 days) and throughout the cycle indicated a failure of spontaneous ovulation and absence of a subsequent luteal phase in this species. High concentrations of testosterone and oestrogens indicated that the oestrous cycle in the camel is mostly follicular and that the increasing values of the two hormones during follicular development (5 days) is probably the stimulus to behavioural oestrus.