Both the age at which housing conditions are imposed and the type of activity measured are shown to affect the results obtained in studies concerned with the effects of social density on the behavior of laboratory rats. In a review of the effects of population density in the cage environment upon the later activity of laboratory rodents, Archer (1970) notes that many of the studies have produced conflicting results. While some investigations have shown isolates to be less active than grouped animals in a novel test situation (Stern, Winokur, Eisenstein, Taylor, & Sly, 1960; Moyer & Korn, 1965; Archer, 1969), others have shown the reverse to be true (Essman, 1966; Weltman, Sackler, & Sparber, 1966). Syme and Hughes (1972) and Syme, in two studies comparing the activity of isolated and grouped rats, suggested several reasons for these discrepancies. Firstly, cage size of isolated animals was shown to affect activity in the open field, and secondly, it was demonstrated that rats and mice should be considered separately since activity differences arising from grouped and isolated housing conditions are opposite for the two species within 1 day of weaning. There are, however, a number of other contributing variables, two of which arc investigated in this study. It is possible that the age at which housing conditions arc introduced is important. Bronfenbrenner (1968) suggests that the effects of stimulus deprivation are negatively related to the age at which such deprivation occurs. The present experiment was designed primarily to investigate the effect of imposing different housing conditions at weaning rather than at some later time. In contrast to Archer (1969), who isolated female rats at 40 days of age and compared their activity with that of grouped animals 24 weeks later, this study used female rats isolated at weaning and tested them after the same period in comparison with control-grouped and crowded animals. Adrenal weights, as a supplementary index of social stress (Schnurer, 1963; Barnett, 1964), were obtained after 3 and 6 months. Two measures of activity are used. Locomotor activity was observed in an open field, while nonspecific general activity was measured on an activity platform. This provides a test of the prevalent assumption (Baumeister, Hawkins, & Cromwell, 1964) that the different methods can be regarded as equivalent when comparisons of activity studies arc made.