Ovariectomized female rats were implanted with estradiol containing silastic implants to induce constant circulating levels of the steroid, and sacrificed every 2 h in order to determine neuroendocrine rhythms. Under these conditions, we observed very marked circadian fluctuations in the hypothalamic concentrations of corticotropin-releasing factor (CRF). luteinizing hormone-releasing hormone (LHRH) and thyrotropin-releasing hormone (TRH), and in plasma levels of adrenocorticotropin (ACTH), corticosterone, luteinizing hormone (LH) and prolactin; the amplitude of the prolactin cycle was in particular much higher than in non-chronically estrogenized animals. The daily variation in CRF, ACTH and corticosterone showed significant rank correlations. Changes in hypothalamic content of LHRH and TRH were biphasic; the increase observed during the light period was abruptly interrupted by a depletion episode, coincident with the period of maximal LH and prolactin secretion, respectively. The initial phase of ACTH, LH and prolactin increments occurred between 11.00 and 15.00 h, and was relatively well synchronized. The steepest rise in ACTH and prolactin occurred at the same time, and preceded that of LH by a constant lag of about 2 h. After that initial period, secretion kinetics of the three hormones followed an independent pattern. The data suggest that increased secretion of several hormones results from activation of neural mechanisms occurring within a limited period of the 24-hour cycle.