Multiple-unit activity was recorded from the basal hypothalamus and preoptic region in proestrous rats, while EEG, ECG, and respiration were monitored under urethane anesthesia at a dosage that blocked spontaneous ovulation. Under these conditions, on the evening of proestrus, rapid of the vagina and uterine cervix with a glass rod induced a localized elevation of the multiple-unit activity in the medial preoptic nucleus and the median eminence. The latency to onset of the rise in activity was approximately 20 min in both regions; maximal levels were reached in 1½ h and a return to baseline was seen within 3 h. Vaginal stimulation did not induce this characteristic change in diestrous rats, nor did anal stimulation in proestrous animals. A brief rise in serum LH levels was seen only after a significant increase in multiple-unit activity. The results are consistent with the hypothesis that the reflex discharge of LH in the proestrous rat involves preoptic activation of the basal hypothalamus.