In the course of screening for hepatoprotective agents from natural products, the effects of the methanol extract (ME) of the rhizome of Alisma orientale (Alismataceae) and its major component, alisol B 23-acetate (ALB) on hepatic lipid peroxidation and drug-metabolizing enzymes were evaluated in rats intoxicated with bromobenzene (BB). Pretreatment with ME and ALB had no effect on hepatic antioxidant enzymes such as glutathione reductase and a-glutamylcysteine synthetase. ME and ALB had also no effect on the reduction in glutathione content caused by BB. In contrast, ME recovered the BB-induced decrease in epoxide hydrolase and glutathione S-transferase, enzymes that remove toxic epoxides. ME also attenuated the BB-induced increase in aminopyrine N-demethylase and aniline hydroxylase, enzymes that produce toxic intermediates. This effect was greater than that seen with ascorbic acid, which was used as a positive control. ALB had similar effects on the activities of antioxidant enzymes to ME, and may be partly responsible for the effects of ME.