Effect of ethanol (20% in drinking water) or acetone (1% in drinking water) treatment was investigated on N-diethylnitrosamine (DEN), acetyl-aminofluorene (AAF) and partial hepatectomy (PH) induced hepatic tumors in rats. Simultaneously with the morphological detection of foci and nodules in the liver of the sacrificed rats, the activities of isozymes of cytochrome P450IIE gene subfamily responsible for the oxidation of ethanol or acetone (as aniline hydroxylase) and also the activity of aminopyrine N-demethylase were determined. Nodules could be detected after DEN, AAF and PH treatment with and without combination with acetone, however nodules did not developed in ethanol treated animals even 6 months after the DEN injection. As expected acetone or ethanol selectively increased the activity of aniline hydroxylase without a general induction of P-450 enzymes. It is suggested that the induction of P-450IIE isoenzymes per se is not connected to the preventive effect of ethanol on DEN induced carcinogenesis.