The effect of the ureic herbicide Linuron [3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea] on the levels of some hepatic xenobiotic metabolizing enzymes was studied in rats. The cytochrome P450-dependent monooxigenase activities of aryl hydrocarbon hydroxylase (AHH) and of aminopyrine N-demethylase (APD) were measured in rat livers after a 14-d treatment by gavage with Linuron. AHH was employed as a marker of the catalytic activity of P450IA1 and APD as a marker of the catalytic activity of P450IIB1/2. Furthermore, the enzymatic activities of the cytosolic via glutathione detoxifying enzymes glutathione peroxidase and glutathione S-transferase were assessed. Three doses of Linuron (both as pure compound and as commercial preparation) were tested. The doses tested were 150, 300, and 450 mg/kg body weight for the pure compound and 315.8, 631.6, and 947.4 mg/kg for the commercial preparation. Differences were found in the relative liver weight only in rats treated with the commercial formulation. The aryl hydrocarbon hydroxylase activity was increased with all the tested doses of pure and commercial Linuron. A reduction in the aminopyrine N-demethylase activity was noted for the highest dose of pure Linuron, whereas an increment in this activity was observed for all the doses of the commercial preparation tested. The activity of glutathione peroxidase was not affected by treatment with the pure product; however, an increment in activity was observed at all the tested doses of the commercial preparation. The glutathione S-transferase activity was reduced in both cases.