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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.