In order to investigate the primary site of action of estradiol, whether pituitary or hypothalamic, gonadotropin responses to estrogen were studied in female rhesus monkeys before and immediately after pituitary stalk section. The estrogen challenge, consisting of either an injection of estradio benzoate (400 microgram) or an implant of three silastic capsules containing 17 beta-estradiol, was initiated on days 2--5 of the menstrual cycle. The estrogen was given not later than 8 h after stalk section. Estrogens induced LH surges in all five animals before and after stalk section. FSH increases were observed in four of five intact and three of four stalk-sectioned animals. Mean FSH and LH levels in three stalk-sectioned animals treated with oil alone did not differ significantly from preinjection controls. These experiments suggest that the locus of estrogens on gonadotropin release in the rhesus monkey may well reside within the pituitary gland itself.