Positive feedback reactions were induced in five female volunteers with regular menstrual cycles on the pituitary secretion of luteinizing hormone (LH) during the follicular phase of the cycle by the i.m. administration of oestradiol benzoate ('oestradiol study'), or by oestradiol benzoate followed by progesterone ('oestradiol/progesterone study'). Prior to the administration of the steroids (basal profiles) and during the LH surges following the administration of the steroids (stimulation profiles), blood was drawn at 10-min intervals and the LH pulsatility assessed. The LH pulses occurred at hourly intervals in basal and stimulation profiles, which concurs with data obtained during the normal proliferative phase and the spontaneous mid-cycle surge of the cycle in both the human and the rhesus monkey. Our findings are, however, in sharp contrast with those obtained in the rhesus monkey by the determination of hypothalamic multi-unit activity.