The aim of the present study was validating pharmacologically a mouse model of alcohol addiction. Mice (n = 60) were offered ethanol (5% and 10%) and water in a free choice paradigm consisting of four phases: free choice (10 weeks), withdrawal (2 weeks), re-exposure (2 weeks) and quinine- adulteration (2 weeks). Control mice (n = 10) had access to water. They were housed individually with food ad libitum. The animals' behaviour was evaluated at the beginning of the treatment and during the withdrawal period. After the exposure to the model, mice received i.p. naltrexone (0.0; 0.125; 2.0 and 16.0 mg/kg) or saline. Mice were characterized as: addicted (n = 15, preference for ethanol without reducing intake when ethanol were adulterated with quinine); heavy drinker (n = 14, preference for ethanol but reduced intake when ethanol were adulterated); and light drinker (n = 16, no preference for ethanol). Naltrexone reduced ethanol intake in the heavy and light groups (p <or= 0.01 and p <03= 0.05, respectively, compared to saline-treated group) with no effect on water intake. It is discussed that naltrexone may be acting in the positive reinforcing properties of ethanol but does not seem to have anti-craving properties. It was concluded that the addicted mice had a compulsive behavior manifested by the continued ethanol intake even under aversive conditions and under naltrexone treatment suggesting that this model might be useful to study addiction.