The effects of anterior and complete hypothalamic deafferentation on prolactin secretion in the rhesus monkey have been assessed. Complete disconnection ofthe medial basal hypothalamus (MBH) had no apparent effect on serum prolactin concentrations, in either intact or ovariectomized monkeys, nor did it diminish the stimulation of prolactin secretion induced by sodium pentobarbital anesthesia. Anterior disconnection of the MBH in intact females was also without effect on prolactin secretion. When the MBH was inadvertently damaged during the complete disconnection procedure, serum prolactin levels increased approximately 5-fold in otherwise intact animals while remaining unchanged in ovariectomized monkeys, thereby suggesting that ovarian hormones may modulate the secretion of prolactin in response to a reduction of hypothalamic inhibitory influences. In contrast, basal serum prolactin concentrations were not notably influenced by ovariectomy or estrogen administration nor were the patterns of prolactin secretion related to the phases of the menstrual cycle. The results of these studies suggest that the consequences of complete MBH disconnection on prolactin secretion are essentially the same in the rhesus monkey and in the rat, although major differences exist between these species with regard to the influence of estrogen on the production of this hormone.