The sexual behaviour of ovariectomized female hamsters was examined after treatment with oestrogen alone and after the addition of progesterone. Both the proceptive and receptive components of behaviour were recorded at four oestrogen dose levels; progesterone dose was kept constant. Oestrogen alone caused dose-dependent increases in proceptive behaviour and, at the higher dose levels, a small increase in receptivity. The addition of progesterone increased proceptive behaviour in females primed with the lower dose of oestrogen but not the higher. Receptivity was markedly increased in all animals but the intensity of lordosis was oestrogen-dose dependent.