Barraclough & Gorski (1962) concluded from their experiments on female rats that the residual effects of an injection of progesterone may facilitate the induction of sexual receptivity when oestrogen is subsequently administered. Other reports, however, would lead one to expect that the residual effects of progesterone treatment would be inhibitory rather than facilitating, since progesterone is said to exert biphasic effects with brief facilitating and prolonged inhibition phases (Kawakami & Sawyer, 1959). Present experiments sought to clarify the facilitating and inhibitory effects of oestrogen and progesterone treatment on receptivity. The mature Sprague-Dawley female rats used were ovariectomized 1–4 weeks before the first hormone treatment. All hormones were given s.c. in 0·1 ml. peanut oil. In Expt. I, nine females received a total of 4 μg. oestradiol benzoate on days 1–3 and 0·5 mg. progesterone on day 4, on four consecutive weeks. After the progesterone treatment, each female was tested with