Neuronal activity in the mediobasal hypothalamus was recorded during the steroid-induced surge of luteinizing hormone (LH) in ovariectomized female rats by means of multiunit activity (MUA) recording techniques. The day of subcutaneous implantation of estradiol-containing silastic tubing, which was done at 12.00 h, was designated as day 1, and progesterone (1 mg) dissolved in sesame oil was injected at 12.00 h on day 5. The hypothalamic MUA and serum LH level were monitored between 10.00 and 20.00 h on days 1, 4 and 5. All 5 animals examined showed characteristic increases (volleys) in MUA associated with pulsatile LH secretion. After implantation of estradiol on day 1, gradual and significant decreases in the serum LH level and in MUA volley frequency were observed. In the afternoon of both days 4 and 5, an LH surge occurred, though it was much greater on day 5 than day 4 due to progesterone injection on day 5. However, neither MUA volleys nor considerable changes in baseline MUA were discernible throughout the recording period on days 4 and 5. More than 5 days after the removal of estradiol-containing tubing, MUA volleys were found to reappear in all the animals. These results suggest that estradiol has an inhibitory effect on gonadotropin-releasing hormone (GnRH) pulse generator activity, and that the GnRH pulse generator is not involved in inducing the LH surge in the rat.