In freely-moving female rats multiple unit activity(MUA) was recorded from medial hypothalamic and septal structures for 4–7 days throughout the estrous cycle. A consistent elevation of MUA during paradoxical sleep (PS) was superimposed on a clearcut circadian rhythm with maximal acti-vity during the night and minimal activity during the day. The circadian rhythm of MUA did not show variations related to the phases of the 4-day estrous cycle and was not altered either by ovariectomy or by administration of exogenous estrogen. In contrast, the diurnal cycle of MUA was completely abolished within 3 weeks following adrenalectomy. It is suggested that the circadian rhythm of hypothalamic MUA in the female rat may be related to the rhythm of secretion of ACTH and/or hypothalamic corticotropin-releasing-factor (CRF).