Hepatoprotective effects of methanol extract and alisol B 23-acetate of Alisma orientale were studied in acetaminophen (APAP)-treated rats. APAP increased hepatic content of lipid peroxide, which was suppressed by methanol extract and alisol B 23-acetate. The liver of rats treated with APAP had higher P-450, aminopyrine N-demethylase and aniline hydroxylase activities than those of normal control rats. The increases in hepatic drug metabolizing enzymes by the i.p. injection of APAP were significantly alleviated by the administration of methanol extract or alisol B 23-acetate. The injection of APAP also resulted in a substantial reduction of hepatic glutathione content and glutathione S-transferase activity, and the decreases were partially, but significantly, restrained by the oral administration of methanol extract prior to the i.p. injection of APAP. Hepatic activities of glutathione reductase (GR) and ${gamma}$-glutamylcystein synthetase ${gamma}$-GCS) were also decreased significantly in APAP-treated rats. The decreases in hepatic GR and ${gamma}$-GCS activities by APAP injection were improved partially, but significantly, with administration of methanol extract of A. orientale. Treatment with alisol B 23-acetate also improved the hepatic ${gamma}$-GCS activity significantly, but not GR.