Methylenedioxybenzenes (MDBs) and structurally related alkenylbenzenes were compared for their effectiveness in preventing carbon tetrachloride (CCl4)-induced liver necrosis in mice. Pretreatment with isosafrole, safrole, dihydrosafrole, and benzodioxole at dosages as low as 10 mg/kg significantly prevented the increase in plasma transaminase levels and histochemical changes associated with CCl4-induced liver necrosis, whereas piperonyl butoxide (PBO), eugenol, isoeugenol, sesamol, and curcumin did not prevent CCl4 hepatotoxicity even at 200 mg/kg. However, isosafrole was only partly hepatoprotective if administered 10 min after the toxicant. Liver microsomes isolated 1 hr after isosafrole but not after PBO administration had a markedly decreased CYP2E1 activity. Isosafrole, safrole, dihydrosafrole, and benzodioxole in vitro also inhibited CYP2E1-dependent metabolism more effectively than eugenol and isoeugenol, whereas PBO did not inhibit CYP2E1 activity. The protective effects of isosafrole, safrole, and benzodioxole were therefore predominantly attributed to their ability to inactivate CYP2E1, the major isozyme involved in CCl4 bioactivation. The marked potentiation of CCl4 hepatotoxicity in CYP2E1-induced mice was also completely prevented by isosafrole but not PBO pretreatment, supporting the hypothesis that CYP2E1 inhibition by isosafrole contributes to its hepatoprotective effect against CCl4. Isosafrole and safrole also prevented bromotrichloromethane (BrCCl3)-induced hepatocyte cytotoxicity, whereas PBO was ineffective.