Changes in the concentrations of ovarian steroids and pituitary gonadotrophins were measured by radioimmunoassay in the jugular plasma of six Clun Forest ewes throughout the oestrous cycle. The concentration of oestradiol began to rise 12-14 h before the onset of oestrus from values of 11-2 +/- 0-36 (S.E.M.) pg/ml during the luteal phase to 21-1 +/- 2-01 pg/ml at -8 to 0 h (oestrus). There was no distinct increase during the luteal phase. Circulating progesterone varied in a cyclic manner with the highest values at the mid-luteal phase (3-70 +/- 0-28 ng/ml; n = 28). In five out of six ewes the concentration was still quite high (1-86 +/- 0-43 ng/ml) at 35 h before the onset of oestrus. The concentration declined rapidly thereafter, reaching minimum values about 12 h before oestrus coincident with the increase in oestradiol concentration. Plasma LH increased from very low values of 2-59 +/- 0-09 ng/ml during the luteal phase to 75-3 +/- 7-4 ng/ml about 9 h after the onset of oestrus. Two peaks of plasma FSH concentration were detected after the onset of oestrus. The first peak (171-0 +/- 35-5 ng/ml) coincided with the LH peak and the second (133-0 +/- 10-7 ng/ml) occurred about 24 h later at a time when LH values were low. The mean FSH concentration at other times during the cycle was 61-9 +/- 2-8 ng/ml.

Summary. Testosterone, oestradiol and progesterone were measured in peripheral plasma during the oestrous cycle of 6 heifers. Oestradiol and progesterone results confirmed earlier reports. Concentration of testosterone on the day of oestrus was 40 \ m=+-\ 3 pg/ml (mean \ m=+-\ S.E.M.), and two peaks were detected during the cycle, one 7 days before oestrus (1809 \ m=+-\ 603 pg/ml) and the other (78 \ m=+-\ 7 pg/ml) on the day before the onset of oestrus. The concentration of progesterone declined in most cases 1 day after the maximum concentration of testosterone.Betamethasone treatment in 5 heifers extended luteal function by an average of 10 days: plasma androstenedione and oestradiol concentrations were unaltered; cortisol values were depressed for at least 16 days after treatment; testosterone concentrations were lowered by 13 \ m=+-\ 2\m=.\4%during treatment, and except in one heifer the peak on Day -7 was abolished.

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Changes in plasma concentrations of hormones and uterine activity associated with spontaneous parturition in the goat were examined. No change in oestradiol-17beta concentration was detected during the experimental period, but oestradiol-17alpha consistently increased in concentration 3-4 days before parturition. This was followed by an increase in prostaglandin F two days later, while a pre-partum decline in progesterone concentration occurred 18-20 h after the significant increase in prostaglandin F. The onset of uterine contractions coincided with a significant decline in progesterone concentration and a period of continually rising prostaglandin F and oestradiol-17alpha concentrations. Uterine activity expressed in Montevideo units did not change until 10-8 h before foetal expulsion. These results support the hypothesis that in the goat the elevated prepartum concentration of prostaglandin F brings about luteolysis and a subsequent decline in progesterone concentration.