Rats with a 4‐day estrous cycle were given a single dose of reserpine (62.5, 125, 500, 1000 or 2000 μg/kg body wt) or chlorpromazine (1, 2.5 or 5 mg/kg body wt) at 13.00 h on the days of estrus, diestrus 1, diestrus 2 or proestrus and were autopsied on the next expected day of estrus. The doses of reserpine (in μg/kg body wt) necessary to block ovulation during the cycle were 500 at estrus, 62.5 at diestrus 1, 500 at diestrus 2 and 2000 at proestrus, and for chlorpromazine (in μg/kg body wt) 2.5 on diestrus 1 and diestrus 2 and 1 in proestrus. On estrus, at 13.00 h, chlorpromazine was ineffective to block ovulation.