Different types of anterior hypothalamic deafferentations have been used to investigate the nervous pathways involved in spontaneous and mating-induced ovulation in the rat. Knife cuts which circumscribed the suprachiasmatic nuclei on all but their ventral surface and either their rostral or caudal poles prevented spontaneous ovulation, but the rats were sexually receptive (copulation plugs and sperm in the smear), and mating induced ovulation. Similar types of cut extended dorsally so as to sever the continuity between the preoptic area and the mediobasal hypothalamus also prevented spontaneous ovulation, and although these rats were also receptive, mating did not induce ovulation. Two possible explanations are considered: either (i) that difference between the effects of the two types of cut is a direct function of the differing proportions of gonadotrophic hormone-containing axons severed, or (ii) cuts in the region of the suprachiasmatic nuclei specifically impair a mechanism for the maintenance of diurnal rhythms, to which the abnormality in gonadotrophin control is secondary.