Plasma LH was significantly increased by electrical stimulation of the median eminence (ME) in ovariectomized and in estradiol-treated sheep. In untreated castrates, spontaneous discharges of LH occurred at regular intervals of approximately one hour (LH rhythm). When ME stimulations were initiated as early as 5–15 min following a spontaneous discharge, they increased LH as much as either ME stimulations initiated at other times or spontaneous discharges. These data failed to demonstrate any pituitary refractoriness to ME stimulation, and presumably to endogenous LH-releasing factor (LRF). Therefore, neural mechanisms involving rhythmic release of LRF were indicated. The chronic administration of estradiol inhibited the spontaneous discharges of LH, and subsequent ME stimulation increased plasma LH even more markedly than in untreated castrate