Electrical activity of the brain during the critical period in the rat was examined throughout the estrous cycle, using chronically implanted macro-micro electrodes. Slight circadian rhythms were observed in the basal level of the integrated multiple unit activity (MUA) in the basal hypothalamus and forebrain limbic area. In addition to these changes, a gradual increase of MUA in the basal hypothalamus was observed during the evening of diestrus day 2, while there was no change in the forebrain limbic preoptic area. However, a characteristic elevated pattern of MUA appeared for 12 to 25 min during the critical period (2 p.m. to 4 p.m.), both in the basal hypothalamus and forebrain limbic preoptic area, i.e., in the arcuate nucleus (ARC), ventromedial hypothalamic nucleus (VMH), medial preoptic area (MPO), septum (SEPT), amygdala (AMYG) and bed nucleus of stria terminalis (BST). The episodes of the characteristic pattern were also observed in the basal hypothalamus on the afternoons of diestrus day 2 and estrus, as well as in the forebrain limbic preoptic area on the day of estrus. To determine the possible relationship between the characteristic pattern and gonadotrophin release, electrochemical stimulation was applied to the areas of the brain where the characteristic pattern was observed during the critical period. Ovulation could be induced by stimulation of the ARC, MPO, SEPT, AMYG, and BST, but not by stimulation of the reticular formation (RF) and hippocampus (HPC). Furthermore, the results shed light on the facilitatory and inhibitory effects of progesterone on ovulation and the onset of the critical period.