SUMMARY Thirty ewes with regular oestrous cycles were divided into six groups for slaughter relative to the time of first acceptance of the ram. The slaughter times were: 0 h (in practice within 40 min of onset of oestrus), 6, 12 and 36 h after onset and days 10 and 16 of the cycle. The hypothalamus was removed and the luteinizing hormone (LH) releasing factor activity extracted with 0·1 m-HCl. The extracts were tested for LH releasing activity by adding them to the medium in which anterior pituitary tissue from castrated male sheep was incubated. The LH content of the medium was measured by the ovarian ascorbic acid depletion method (Parlow, 1958). The activity of the extract from the group slaughtered on day 16 of the cycle was high (minimal effective dose (MED) = 0·00625 hypothalamic equivalents (HE)). The potency declined with the onset of oestrus and remained low at 6 and 36 h after onset (MED in each case 0·025 HE) with intermediate potencies at 12 h and 10 days after onset (MED in each case 0·0125 HE). These changes are compared with changes in the LH content of the pituitary gland (bioassay) and of the plasma (radioimmunoassay) and with parameters of the ovarian activity of the animals.

A sheep pituitary incubation system was developed which may be used to study the release of both luteinizing hormone (LH) and follicle-stimulating hormone (FSH) as assayed biologically. Specificity was examined by adding at high concentration various materials present in the hypothalamus, a crude acid extract of sheep cerebral cortex or crude or partly purified extracts of ovine hypothalamic tissue.Of the materials tested for LH and FSH releasing activity, oxytocin, adrenaline, synthetic lysine vasopressin and cerebral cortex extract failed to influence LH or FSH release. Natural vasopressin containing unknown proportions of the lysine and arginine forms did not affect FSH release but produced a significant increase in the LH content of the medium, the reason for which was not clear. Noradrenaline failed to influence FSH release but produced an apparent depression in the LH content of the medium due to an action on the LH released or upon the ovarian ascorbic acid depletion assay since the addition of noradrenaline to standard ovine LH reduced the potency in the assay to an almost identical degree.Extracts of ovine hypothalamic tissue consistently increased the LH and FSH content of the medium.The method described may be applicable to the detection and estimation of LH and FSH releasing activity in ovine hypothalamic tissue extracts and to detecting changes in the responsiveness to hypothalamic stimulation of sheep pituitary tissue.