Pregnant rats were injected subcutaneously with either 1 μg oestrone, 12·5 mg Depo-medroxyprogesterone acetate (MPA), or both. Oestrone injected on Day 3 had no effect on pregnancy. Injection of MPA on Day 1 caused delayed implantation, which was abolished when, in addition, oestrone was injected on Day 4. Injection of MPA on Day 1 followed by oestrone on Day 3 destroyed the eggs during the morula—blastocyst transformation period. The contraceptive effect of the latter treatment suggests a general guide-line for postcoital contraceptive trials in women.