The purpose of this investigation was to compare the self- administration of heroin and cocaine base, alone and in combination, in rhesus monkeys (Macaca mulatta) self-administering a combination of heroin (0.1 mg/kg/delivery) and cocaine base (1.0 mg/kg/delivery) via the smoking route. Smoke deliveries were contingent on completion of a chained fixed ratio (FR; lever press), FR 5 (inhalation) schedule. The lever press FR values (64, 128, 256, 512, and 1,024) represented increasing drug price. Demand functions (Consumption x Price) were obtained for the heroin and cocaine combination and compared with previously determined demand functions for smoked heroin and cocaine alone. As the FR increased and the number of responses emitted increased, the number of drug deliveries decreased. The demand functions were not different for heroin versus cocaine alone or for cocaine alone versus the cocaine-heroin combination. However, the demand for heroin alone was significantly less than the demand for the cocaine-heroin combination, suggesting that smoked cocaine base enhances the behavioral effects of smoked heroin.