SUMMARY The effect of injections of progesterone on the length of the behavioural oestrous cycle in rats is described. Administration of the hormone was shown to act differently in rats known to have a 4-day cycle as compared with animals with a 5-day cycle. In the 4-day rat injection of 2 mg. progesterone on the first, second or third day of the cycle retarded the next occurrence of behavioural oestrus by 1–2 days; treatment on day 4 of the cycle was without effect. In 5-day rats treatment with progesterone before day 3 of the cycle delayed the occurrence of behavioural heat in only 35% of animals; treatment on day 3 retarded the occurrence of heat by 1–3 days in all rats. Injections on day 4 advanced heat by 1 day and treatment on day 5 was without effect. Additional experiments compared the responsiveness to progesterone of 'artificial' and natural 5-day rats. The results suggest a biphasic effect of progesterone on induction of behavioural receptivity with time characteristics similar to those observed earlier for ovulation and vaginal cyclicity.