Ovariectomized rhesus monkeys bearing hypothalamic lesions which had abolished endogenous LHRH production, as evidenced by a profound reduction in gonadotropin secretion, but in which LH and FSH secretion was reestablished by a chronic intermittent iv infusion of synthetic LHRH (1 microgram/min for 6 min every hour) were used to investigate the sites of the negative and positive feedback actions of estradiol in the control of gonadotropin secretion. The administration of estradiol to such animals, while continuing the LHRH replacement regimen, resulted in a decline in circulating LH and FSH levels, followed by an unambiguous discharge of these hormones. The time course of this biphasic pattern of gonadotropin secretion was remarkably similar to that observed in response to estradiol administration in otherwise intact ovariectomized animals. These results suggest that, in the rhesus monkey, estradiol can exert both its negative and positive feedback actions on gonadotropin secretion at the level of the pituitary gland.