In rhesus monkeys, localized lesions in the anterior hypothalamus (above the optic chiasm) blocked the preovulatory release of LH and the subsequent luteal phase increase in serum progesterone. Electrical stimulation of this same hypothalamic region in ovariectomized monkeys released significant LH and prolactin without visible behavior effects. Stimulation more caudally in the medial-basal hypothalamus (MBH) also released LH, FSH, and prolactin. The magnitude of hormone release was proportional to the strength of current applied to the MBH and 48 to 96 hours of prior estradiol treatment diminished the quantity of hormone released. Rostral neural pathways into the MBH appear to influence gonadotropin release and the MBH-hypophyseal axis is one source of negative feedback by estradiol. In other studies we have found that systemic prolactin concentrations in female rhesus are higher at night than during the day. Prolactin concentrations are not influenced by stage of the menstrual cycle or copulation; they can be affected by drugs, stress, and anesthesia. BIBLIOGRAPHIC REFERENCES: Quadri, S.K., and Spies, H.G. Cyclic and diurnal patterns of serum prolactin in the rhesus monkey. Biol. Reprod., 1976, in press. Tang, L.K.L., and Spies, H.G. Effects of hypothalamic-releasing hormones on LH, FSH, and prolactin in pituitary monolayer cultures. Proc. Soc. Exp. Biol. Med., 1976, in press.