Effect of intraperitoneal (IP) injection of naltrexone (Ntx), at a dose ineffective per se (.25 mg/kg), on buprenorphine-induced increased ethanol intake (.03 mg/kg) in Marchigian Sardinian alcohol-preferring rats. Buprenorphine (Bup) was injected IP 5 min after Ntx administration, and ethanol was given to the animals 90 min after Bup. Alcohol consumption was monitored at 30, 60, 90, and 120 min. Values represent the mean (±SEM) of 10 subjects. Difference from control subjects: *p < .05. Veh, vehicle.