Department of Endocrinology, Growth and Reproduction, Faculty of Medicine, Erasmus University, P.O. Box 1738, Rotterdam, The Netherlands (Received 14 June 1978) Prenatal morphological and behavioural masculinization of female foetuses through exposure to androgen from the testes of male foetuses was first suggested by Clemens (1974). It was reported that female foetuses next to male foetuses in the uterus had a larger ano-genital distance at birth and displayed more (but not statistically significant) mounting behaviour in response to administration of testosterone in adult life than did female foetuses that had not been in such close proximity to foetal male rats. More recently, Gandelman, vom Saal & Reinisch (1977) also reported that in mice, the positions of the female foetuses in the uterus relative to the male foetuses determined the extent to which the female rat was masculinized in terms of both morphology (ano-genital distance at birth) and behaviour (testosterone-induced fighting in