The effects of Glycyrrhizae Radix (GR) on the metabolism of acetaminophen (AA) were examined in male Sprague-Dawley rats. The methanol extract of GR (500 mg/kg) was administered orally to rats for 6 days. AA and its metabolites excreted in bile, urine and blood within 120 min after dosing of AA (150 mg/kg, i.v.) were assayed by HPLC. Treatment of rats with the methanol extract of GR significantly increased the cumulative biliary excretion of AA-glucuronide (156% of the control) and decreased that of AA-sulfate (63% of the control). The cumulative urinary excretion of AA-glucuronide was also significantly increased to 132% of the control. GR treatment significantly increased total (biliary plus urinary) excretion of AA-glucuronide (172% of the control) without influencing thioether and sulfate conjugates of AA. The results clearly show that GR enhances UDP-glucuronosyl transferase-mediated detoxication of AA, but may not influence sulfotrans-ferase-mediated and cytochrome P-450-mediated metabolites formation.