In the following series of experiments, effects of morphine (0.1, 0.3, 1.0, 3.0, and 10.0 mg/kg) and naloxone (0.1, 0.3, and 1.0 mg/kg) were assessed in nondeprived rats trained to leverpress with 10% ethanol, sweetened ethanol, or 5% sucrose and water as the reinforcers. Morphine, at doses of 0.1, 0.3, and 1.0 mg/kg had little effect on responding with ethanol or sweetened ethanol available on a fixed ratio 4 (FR4) schedule of reinforcement, but at the 3.0 mg/kg dose, morphine suppressed responding to near zero. Similar results were obtained when 10% ethanol and water were available on a concurrent FR4 FR4 schedule of reinforcement. When 5% sucrose and water were available concurrently, morphine suppressed responding at 3.0 and 10 mg/kg. Naloxone (0.1, 0.3, and 1.0 mg/kg) decreased responding for ethanol, sweetened ethanol, and sucrose solutions in a dose-dependent manner. Naloxone decreased total number of responses/session by shortening the duration of responding without affecting momentary rate. Overall, the data suggest that the endogenous opioid system plays a role in the ability of ethanol to reinforce operant behavior. However, this role does not appear to be specific to ethanol because similar results were observed with sucrose reinforcement. Failure to find enhanced ethanol intakes following morphine injections in the operant situation suggests that the method used to measure ethanol self-administration makes a difference in assessing the effects of drugs on ethanol intake.