Thirty-nine families were observed extensively at home when children were 2 1/2 and 4 1/2 years of age and again 2 years later. The Social Relations Model is used to investigate children's sex differences in aggression and parents' prohibiting aggression during sibling conflict. In the first observation period, boys engaged in more severe and mild physical aggression, grabbing, insulting, and property damage than girls. At Time 2, boys engaged in more mild physical aggression and insulting, than girls, but there were no sex differences in other forms of aggression. At Time 1, parents' responses were relatively uninfluenced by the sex of the children. However, parents were more likely to prohibit mild physical aggression, grabbing, and property damage by and towards girls at the second time period. Conversely, parents showed more tolerance for boys' mild physical aggression at Time 2, suggesting that this socialisation message may play a role in boys' greater use of physical aggression both at home and with peers. (Contains 5 tables.)