SUMMARY

Changes in the concentrations of ovarian steroids and pituitary gonadotrophins were mea-sured by radioimmunoassay in the jugular plasma of six Clun Forest ewes throughout the oestrous cycle. The concentration of oestradiol began to rise 12\p=n-\14 h before the onset of oestrus from values of 11\m=.\2\m=+-\0\m=.\36(s.e.m.) pg/ml during the luteal phase to 21\m=.\1\m=+-\2\m=.\01pg/ml at \m=-\8to 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\m=.\70\m=+-\0\m=.\28ng/ml; n = 28). In five out of six ewes the concentration was still quite high (1\m=.\86\m=+-\0\m=.\43ng/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\m=.\59\m=+-\0\m=.\09ng/ml during the luteal phase to 75\m=.\3\m=+-\7\m=.\4ng/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\m=.\0\m=+-\35\m=.\5ng/ml) coincided with the LH peak and the second (133\m=.\0\m=+-\10\m=.\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\m=.\9\m=+-\2\m=.\8ng/ml