Effect of intraperitoneal injection of naltrexone (Ntx) (0, .25, 1.0, and 2.5 mg/kg) on ethanol intake in Marchigian Sardinian alcohol-preferring rats. Drug injections were given 95 min before ethanol access, and 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.