In ovine species, males and females from temperate latitudes show seasonal variations in reproductive activity. The quantity and quality of sperm produced by the male decrease during spring. In contrast with the male, the seasonal regulation of reproduction in female sheep appears to be mainly an all-ornothing phenomenon. During spring and summer, the ewe exhibits a complete cessation of activity, both of ovulation and oestrous behaviour. Changes in daylength around the year are responsible for this alteration of reproductive activity. Under constant artificial daylength of 8 or 16 h of light per day, the animals exhibit a neuroendocrine situation of breeding or anoestrous season respectively (Karsch et al., 1986). Photic information is conveyed into the brain through several relays and results in a nocturnal secretion of melatonin. Thus, the duration of nocturnal melatonin secretion, short during long days and long during short days, informs the animal of the time of the year. Melatonin in turn