Extensive lesions of the suprachiasmatic nuclei result in failure of ovulation in the female rat. Damage to adjacent structures (optic chiasma, anterior pole of the arcuate nuclei, anterior hypothalamus, preoptic area) is neither necessary nor, in itself, sufficient to cause failure of ovulation. All anovulatory animals showed a high level of sexual receptivity and some ovulated after mating but few became pregnant. There was no consistent relation between the incidence of ovulation after mating or after progesterone administration and the occurrence of a facilitation of gonadotrophin secretion by progesterone administration to oestrogen-primed animals after ovariectomy. Nor could these responses be correlated with the extent or anatomical location of the lesions. It is suggested that the failure of ovulation may be related to the abolition or disruption of the normal diurnal variation in sensitivity to the facilitatory effects of ovarian steroid hormones.