Male golden hamsters received either 300 μg testosterone propionate or oil on day 1 of life. As adults they were observed (i) when intact (ii) when castrated and (iii) when receiving 1 mg testosterone propionate per day in interactions with intact dioestrous females. Male controls which had received oil neonatally resembled normal untreated males in showing less aggressive behaviour than females. Castration and subsequent androgen administration did not affect aggression in the controls. Conversely, neonatally androgenized males showed comparable levels of aggression to the females with which they interacted. Castration reduced aggressive behaviour in these males, while subsequent androgen administration resulted in them showing significantly more aggression than females. In particular, when intact or receiving androgen replacement, neonatally androgenized males attacked and bit females, a behaviour seldom seen in normal males or in those given oil neonatally. Sexual investigation and following decreased in both groups of males after castration, and increased under androgen replacement. These changes were more pronounced in the neonatally androgenized males, indicating a greater behavioural responsiveness to androgens in this group. It is suggested that the behaviour of the normal male hamster is incompletely 'masculinized' during neonatal development, and the possible adaptive significance of this is discussed.