In an attempt to define the effects of biphenyldimethyl dicarboxylate (DDB) on the lipid peroxidation and oxygen free radical scavenging enzymes activities in mercuric chloride-induced hepatotoxic rats, we studied malondialdehyde (MDA) level and the activities of catalase and superoxide dismutase (SOD) in liver of the rats at 24 hr after the injection of mercuric chloride. Sprague-Dalwey albino rats were injected subcutaneously with mercuric chloride (5 mg/kg) only and mercuric chloride (5 mg/kg) plus. DDB (200 mg/kg/day, p.o) is administered for 4 days prior to 3 days from the injection of mercuric chloride. The group treated with mercuric chloride showed significantly higher MDA level and lower catalase and SOD activities as compared with that of control group. The group treated with mercuric chloride plus DDB showed significantly lower MDA level and catalase activity and higher SOD activity as compared with that of mercuric chloride-treated group. These results suggest that the excessive oxygen free radicals resulting from the depression of superoxide dismutase activity is an important determinant in the pathogenesis of mercuric chloride-induced hepatotoxicity and DDB has antioxidant effects.