Sexual activity declines after castration in all mammalian species studied (Beach 1948, Beach and Pauker 1949, Beach 1970, Grunt and Young 1953, Michael and Wilson 1974, Phoenix, Slob and Goy 1973, Rosenblatt and Aronson 1958). The rate of decline of sexual behaviour is generally slower than that of the function of the accessory sexual glands. Two factors have been suggested to contribute to the maintenance of sexual behaviour for some time after castration: secretion of androgen from the adrenal cortex and sexual experience. However, adrenalectomy does not seem to influence post-castrational behaviour in hamsters (Warren and Aronson 1956), dogs (Schwartz and Beach 1954), rats (Bloch and Davidson 1968), mice (Burge and Edwards 1971), and cats (Cooper and Aronson 1958), and the degree of sexual experience appears to be of little importance for the postcastrational behaviour of rats (Bloch and Davidson 1968, Rabedeau and Whalen 1959) and dogs (Hart 1968). But male cats with sexual experience continue to mate after castration for a considerably longer period than inexperienced cats (Rosenblatt and Aronson 1958). Some experienced animals were able to achieve insertion at the 10th week after castration, whereas none of the inexperienced animals did so after the first week. In conclusion, the importance of experience seems to vary between species, whereas the adrenal androgens are of doubtful significance for postcastrational behaviour. We have casually observed that castrated rabbits retain their sexual vigour longer if they are given extensive precastrational experience. The importance of experience has now been studied systematically in animals of mixed strains generally used in the laboratory. As a comparison, we studied the postcastrational behaviour in some New Zealand White rabbits with extensive sexual experience.