In a previous experiment undernutrition during the suckling period, but not during gestation, was found to have lasting effects on the social behaviour of male rats. To establish whether the sensitive period for this effect extends beyond weaning, rats were undernourished in the present experiment from 30 to 55 days of age. To control for the possibility of competition between the underfed rats for food, one group was caged in pairs and the other singly to obviate competition. All rats were fed ad lib from 55 days and testing began at 100 days. There were no significant effects of nutritional treatment or of method of housing on any aspect of social behaviour. This result, together with that of the previous experiment, indicates that the sensitive period for the undernutrition effect is the suckling period.