The possible reinforcing effect of acetylsalicylic acid (ASA) was studied in a group of rhesus monkeys that had no history of self-administration of drugs. Rates of lever pressing were compared under conditions in which each lever-pressing response resulted in an infusion of saline, an infusion of saline plus delivery of a food pellet or an infusion of ASA (0.4, 1.0, 2.5 OR 5.0 MG/KG /infusion). Responding was engendered and maintained by the delivery of food pellets but not by infusions of saline alone nor by ASA; however, responding was subsequently engendered and maintained in these monkeys by codeine (0.05 mg/kg/infusion). In another group of monkeys that had been trained to respond under a 10-response fixed-ratio schedule of i.v. infusions of codeine, the possible reinforcing effects of aminophenazone, phenylbutazone and of combinations of each of these drugs with codeine were studied. Aminophenazone and phenylbutazone (0.4-5.0 mg/kg/infusion) did not maintain responding previously engendered by codeine. Mixtures of aminophenazone and phenylbutazone with codeine decreased the number of codeine self-administrations. Thus, codeine intake was reduced when aminophenazone and phenylbutazone were added to codeine in the solution to be self-administered. These experiments suggest that antipyretic analgesics are not effective in reinforcing behavior in the rhesus monkey.