Two experiments examine the hypothesis that brain differences found in environmental enrichment are due to differences in social interaction. Both experiments compare enriched versus group housed rats using videotape records of home cage activity, scored with a protocol developed by the authors. Experiment 1 examines social interactions in group and enriched housed rats in the first 30 days postweaning; In Experiment 2 rats were housed in the differential environments from 90 to 120 days of age, an age at which rats have been reported no longer to engage in play; in addition, weights of sections of the brain were obtained at sacrifice and these showed typical patterns of differences among rats from differential environments. Neither experiment revealed any consistent pattern of differences in social interaction, either by chi-square comparisons of overall profiles of social activity or by discriminant analysis applied to behavioral observations. No evidence was found in support of the hypothesis that play is responsible for the effects of environmental enrichment.