The role of progestational agents in controlling the sexual behaviour of female primates has not been systematically studied but is important now that oral progestins are taken by large numbers of women for contraceptive purposes. Progesterone facilitates sexual receptivity in the ovariectomized, oestrogen-primed mouse (Ring, 1944), rat (Boling & Blandau, 1939), guinea-pig (Dempsey, Hertz & Young, 1936), and hamster (Kent & Liberman, 1949): in these forms, progesterone induces psychic oestrus, usually with a latency of a few hours, and acts synergistically with oestrogen. In contrast, rabbits in post-partum oestrus are thrown out of heat by progesterone treatment (Makepeace, Weinstein & Friedman, 1937), and implants of progesterone inhibit stilboestrol-induced oestrus in the spayed ferret (Marshall & Hammond, 1945); in these cases there is a clear antagonism. In the ewe, the manifestation of heat behaviour normally depends on the presence of a regressing corpus luteum as well as on follicular ripening; for