CYP2E1 knockout mice (cyp2e1^−/−) were used to investigate the involvement of CYP2E1 in the development of carbon tetrachloride (CCl₄)-induced hepatotoxicity. Malecyp2e1^−/−and wild-type (cyp2e1^+/+) mice were given a single ip injection of 1 ml/kg (=1.59 g/kg) CCl₄and 24 h later liver injury was assessed by elevations of serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) activities and histopathology. No significant increases in serum ALT and AST activities were observed incyp2e1^−/−mice when compared to wild-type counterparts after CCl₄exposure. No detectable abnormality in liver histology was found incyp2e1^−/−mice after CCl₄exposure. In contrast, CCl₄treatment resulted in 422- and 125-fold increases in serum ALT and AST activities, respectively, in wild-type mice. Consistent with the results of serum ALT and AST activities, severe hepatic damage was noted in livers of wild-type mice, indicating the importance of CYP2E1 in mediating the hepatic damage following CCl₄exposure in these mice. In addition, a dramatic decrease in CYP2E1-catalyzedp-nitrophenol activity and complete loss of immunoreactive CYP2E1 were observed in wild-type mice after CCl₄treatment, suggesting that CYP2E1 was degraded during the process of CCl₄-induced hepatotoxicity. These studies conclusively demonstrate that CYP2E1 is the major factor involved in the CCl₄-induced hepatotoxicity in mice.