A single subcutaneous injection of progesterone (0-4 mg) was given to intact female guinea-pigs on either day 1, 7, 12, 13, 14, 15, or 16 of the behavioral oestrous cycle (day of oestrus is day 0). Injections given on either day 1 or day 7 had little effect, although there was a suggestion that injection on day 7 produced a lengthening of the cycle. Animals injected on either day 12 or day 13 underwent cycles of 16-9 and 18-1 days average length, respectively, which were significantly longer than the mean of 15-8 days for uninjected control females. Injection of progesterone on days 14, 15, or 16 was associated with one of three distinct sequelae: (1) simple prolongation of the cycle associated with a return to spontaneous oestrus 4--7 days later; (2) advancement of ovulation, formation of abnormal corpora lutea, and return to spontaneous oestrus 9--13 days later, and (3) return to spontaneous oestrus 14--16 days after the progesterone injection. These findings suggest that progesterone can cause the release of ovulatory amounts of gonadotropin following a period of endogenous oestrogen conditioning of the gonadotrophic system. If progesterone is administered before oestrogen conditioning is complete, then it inhibits or delays the conditioning process, and spontaneous oestrus and ovulation are postponed.