Direct contact with enriched environment is required to alter cerebral weights in rats.

To test the relative effectiveness of direct vs. indirect interaction with an enriched environment, some rats were housed in groups of 12 in large enriched condition (EC) cages while littermate "observer" (OC) rats were placed singly in small wire-mesh cages within EC. A third group was housed singly in an impoverished condition (IC) where stimulation was minimal. After 30 days, the animals were killed and the brains dissected. In both experiments the usual pattern of EC-IC differences in brain weights appeared, whereas OC showed no significant differences from IC. On measures of exploratory behavior taken during the last 2 days of the second experiment, IC fell significantly below EC, and OC was somewhat below IC. Thus EC differen from both IC and OC in brain and in behavior. Active contact with an enriched environment appears necessary for development of EC effects.