In rats injected with depo-medroxyprogesterone acetate (depo-MPA) at either 10.00 or 22.00 hours on the last day of dioestrus, the anticipated ovulation was blocked; injection at 10.00 hours on the day of pro-oestrus did not block ovulation. When ovulation was blocked, there followed a dioestrous period which lasted, on average, 72·3±1·9 days. Following the prolonged dioestrus, the lengths of the initial cycles were irregular. In most cases, mating did not occur during the first oestrous period. When mating eventually occurred, it was fertile yielding normal offspring. In principle, the results are similar to those obtained for women injected with depo-MPA to effect contraception.