Delivery by Caesarean Section (CS) of pregnant rats at 09.00 hours on the 23rd day of gestation reduced the range of times of ovulation after parturition by 18 hr. Ovulation had not occurred by 15 hr after CS but was completed by 18 hr, an interval which is similar to that of ovulation for females of this colony littering at 09.00 hours. Operation on Day 21 or 22 post coitum did not change the day of ovulation (Day 24) but CS on Days 19 or 20 advanced ovulation to Days 22 or 23, respectively. Prevention of normal delivery by ligation of the uterine horns on Day 22 resulted in no females ovulating on Day 24, and few females ovulating on Days 25 and 26. In such females, CS did not increase the number of rats ovulating.