It has been known for many years that the annual breeding cycle of the sheep is controlled by photoperiod. More recently it has become apparent that this process involves the pineal gland. Light is effectively monitored by retinal photoreceptors within the eye which transmit a neural signal to the pineal gland, and this in turn responds by secreting melatonin during the period of darkness. As daylength decreases in the autumn, the duration of elevated melatonin secretion increases, and this changing ratio of high:low melatonin during each 24h period stimulates breeding activity.

Both timed (by afternoon feeding or injection) and continuous (by subcutaneous or vaginal implant) administration of exogenous melatonin to ewes in mid-summer have recently been shown to mimic the effects of decreasing photoperiod by advancing the onset of the breeding season. The present experiment was undertaken to investigate the ability of a subcutaneous implant of melatonin to manipulate reproductive activity of ewes under field conditions in the U.K.