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.

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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.

Three groups of Suffolk-cross ewes were kept in (A) summer photoperiod plus melatonin feeding in such a way as to mimic the plasma levels found in winter photoperiod, (B) winter photoperiod or (C) natural light/dark from mid-June onwards. Prolactin levels remained high in group C throughout July and August but were dramatically reduced in both groups A and B. The rise in prolactin levels associated with dusk, however, was still apparent in all three groups. Appropriate administration of melatonin can thus influence prolactin secretion in the same way as an extension of the dark phase. This effect is associated with an early onset of the breeding season in the ewe.

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