Interactions between the effects of cocaine and caffeine have been demonstrated in tests of motor activity and drug discrimination. Since both drugs are widely consumed by humans, the present study was undertaken to determine whether a similar interaction between the reinforcing effects of these drugs could be demonstrated. Experienced cocaine self-administering rats were treated with caffeine either as an i.p. injection (20.0 mg/kg) prior to each self-administration test or as a co-administered drug within the infusion syringe (0.25 mg/kg/infusion). Both of these routes of administration of caffeine increased the intake of low doses of cocaine. Since caffeine is not reliably self-administered by laboratory animals, these data suggest that caffeine potentiated the reinforcing effects of cocaine.