Preschoolers' prosocial and aggressive behaviours were explored longitudinally, with a focus on the inferred underlying motives of these behaviours. Forty-four children (initially 22-40 months of age) were observed in naturalistic interactions with peers, during a 2-month period, for each of three consecutive years. Three categories of prosocial behaviour (requested, altruistic, and nonaltruistic) and three categories of aggressive behaviour (reactive, proactive instrumental, and proactive hostile aggression) were explored for: (1) internal consistency; (2) developmental changes; (3) individual stability; (4) gender differences; and (5) interrelations. Internal consistency was moderately high for aggression and low for prosocial behaviour. All types of prosocial behaviour were enacted with increasing frequency as children grew older, whereas no developmental changes were revealed for the enactment of aggressive behaviour. Individual stability was found for aggression and for prosocial altruistic behaviour. A single gender difference was found: Girls outperformed boys on altruistic behaviour at the end of the preschool period. Patterns of intercorrelations indicated that (1) prosocial requested behaviour was unrelated to aggression; (2) prosocial altruistic behaviour was negatively related to aggression, in particular to proactive hostile aggression; (3) prosocial nonaltruistic behaviour was sometimes positively related to aggression. The theoretical significance of focusing on underlying motives rather than on behavioural forms is discussed. (Contains 3 tables and 1 footnote.)