To know the mechanism of biphenyl dimethyl dicarboxylate (DDB) in the protection of chemically induced hepatotoxicity, the activity of glutamic pyruvic tran.saminase (GPT) and the level of lipid peroxidation metabolite (malondialdehyde, MDA) and ATP content in hepatocytes were determined in serum and primarily cultured hepatocytes. For in vibo study, rats were pretreated with DDB (300 mg/ kg, p.o.)for 7 days. DDB pretreatment efficiently reduced the elevation of serum GPT activity induced by carbon tetrachloride (1.6 ml/kg, s.c.) and acetaminophen administration (1500 mg/kg, i.p.). In ex vivo study, hepatocytes were isolated from the rats pretreated with DDB (300 mg/kg, p.o.)for 7 days and cultured for 12 hrs before inducing cytotoxicity with chemicals. The MDA formation and the GPT release induced by adriamycin $(1 imes10^{-4} mg/ml)$ and cisplatin $(2 imes10^{-4} mg/ml)$ were markedly decreased in the hepatocytes from the rats pretreated with DDB as compared to vehicle only. However, DDB pretreatment did not prevent the decrease of ATP contents of hepatocytes induced by cisplatin and adriamycin. In in vitro experiment, DDB was pretreated in primary cultured hepatocytes for 3 days. DDB enhanced the decreases of ATP contents induced by cisplatin and adriamycln. These results suggest that DDB may protect the hepatocytes from injury induced by hepatotoxlcants through inhibiting the lipid peroxidation.