The effects on maladaptive behavior and test performance of stark versus enriched physical environments were examined with 16 severely and moderately retarded adults in a large residential institution. The relevant literature was reviewed and classified in four categories: cognitive development studies; studies of the effects of manipulated environmental variables on maladaptive behavior of the retarded; studies of environmental determinants of behavior in naturalistic settings; and studies of environmental variables affecting test performance. The eight male and eight female Ss were given a 20-minute standardized test in both a sparsely furnished room and an enriched, pleasantly furnished room while an observer recorded behaviors. Results suggested that physical decoration of institutional living areas, although welcome as a normalizing influence, may not be effective in significantly decreasing maladaptive behavior or increasing test performance. It was also found that Ss in the enriched setting appeared to engage in less self-stimulatory behavior. (JW)