The timing of the release of ovulating hormone in the rat was studied by means of pentobarbital blockade of ovulation in chronic lighting schedules of 12, 14, or 16 hr of light per 24 hr. In each lighting schedule, pentobarbital (30 mg/kg) was administered to between 10 and 20 animals at 1, 2, 3, or 4 PM on the day of proestrus; the animals were killed the next day and examined for presence of ova. Although the time the lights went on and off in the three lighting schedules was different, the time of maximum ovulation blockade always occurred at 14 hr after midnight (midpoint of dark) or 2 hr after noon (midpoint of light), which corresponds to 2 PM in each group. When animals were studied in 12 hr of light per day but with the lights going on and off 3 hr earlier than in the other groups, the peak of ovulation blockade also occurred 3 hr earlier. It appears that the onset of the "critical period" for ovulating hormone release is not timed by the onset of light or dark, but by the midpoint of either dark or light.