Patterns of plasma melatonin, similar in the duration of high levels to those found in winter, were induced in Suffolk-cross ewes kept in summer light (16 h light: 8 h darkness) by daily oral administration of melatonin (3 mg/13 mumol). The onset of oestrous cycles in these sheep occurred in August, 2-8 weeks before the onset of oestrous cycles in untreated ewes kept in natural light. The onset of oestrous cycles in a further group of ewes kept in winter light (8 h light: 16 h darkness) from mid-June was indistinguishable from that of the melatonin-treated ewes. Rams were excluded from the premises. These data indicate that melatonin alone in physiological quantities is sufficient to induce early onset of the breeding season in the ewe, and provide strong evidence for a hormonal role of melatonin in a short-day breeder.

Summary. Six ewes were ovariectomized in late November and kept in natural light with 5 intact controls. LH levels were monitored approximately weekly. On Days 16 and 58 after ovariectomy plasma melatonin was determined at 3-h intervals for 24 h in all the ewes. Plasma melatonin concentrations were greater in the ovariectomized ewes than in the controls especially at 16 days when total 24 h secretion was almost 3 times that in the control ewes. These data indicate that, in the ewe, the gonads exert an inhibitory influence on melatonin secretion.

The effect of melatonin on the stimulated release of LH from prepubertal female rat pituitary cells in vitro was investigated. Significant inhibition of LH-releasing hormone and calcium ionophore-induced LH release was seen but not of potassium-induced release. These results suggest a specific interaction between melatonin and the endogenous events leading to LH release, and may implicate melatonin as an important neuroendocrine component of pubertal development in this species.