It is known that ethanol increases oxygen consumption in in vitro liver models, which could lead to hypoxia. Although it was shown recently that one large dose of ethanol caused hypoxia in rat liver in vivo, whether ethanol produces hypoxia in a clinically relevant chronic model remains unclear. In the present study, therefore, the effect of chronic ethanol on hypoxia was investigated in vivo using the 2-nitroimidazole hypoxia marker, pimonidazole. Male Wistar rats (300-325 g) were exposed to enteral ethanol continuously for 4 weeks. In this model, rats develop steatosis, inflammation, and necrosis characteristic of early stages of clinical alcoholic liver disease in humans. One hour before they were killed, rats were injected with pimonidazole (120 mg/kg intravenously), and livers were surgically isolated, removed, and fixed. Protein-bound pimonidazole adducts were identified on formalin-fixed, paraffin-embedded tissue with immunohistochemistry. Ethanol administration for 4 weeks significantly increased serum aspartate transaminase levels and hepatic pathology scores for steatosis, inflammation, and necrosis, as expected. Ethanol treatment significantly increased both the extent and number of cells that stained positive for pimonidazole compared with control animals given an enteral diet without ethanol. Quantitative image-analysis of pimonidazole binding showed that 4 weeks of ethanol administration nearly doubled the pimonidazole-positive area in tissue. Ethanol also increased pimonidazole binding significantly at 7 days, long before inflammation and necrosis could be detected. These results indicate that chronic ethanol causes hypoxia at the cellular level in rat liver in vivo and lend support to the hypothesis that hypoxia is involved in mechanisms of early alcoholic liver injury.