Institutionalized mentally retarded, persons (24) received intelligence tests under both standard and reinforcement conditions while on and off psychotropic medication. Medications included chlorpromazine (Thorazine), thioridazine (Mellaril), haloperidol (Haldol), mesoridazine (Serentil) and lithium carbonate (Lithane). On medication there were no differences between IQ obtained under standard and reinforcement conditions. When off medication there were significant increases in scores obtained under both standard and reinforcement conditions. The increase in scores under the reinforcement condition was 23 points greater than the increase obtained under the standard condition (increases of 30.2 and 6.9 points, respectively). The sensitivity-insensitivity of intelligence test scores to medication effects, medications impairing responding to external reinforcement contingencies and legal implications were discussed.