Hepatic drug metabolism in patients with acute viral hepatitis was investigated under different conditions: pregnancy, postpartum, non-pregnancy, and among males. Liver function tests were altered in all of these conditions. The relationship between in vivo and in vitro drug metabolism was studied in twenty-two patients using diagnostic liver needle biopsies by comparing the drug metabolising enzymes (aminopyrine-N-demethylase and bilirubin-UDP-glucuronyl transferase) in these biopsies with the elimination kinetics of antipyrine. Drug clearance tests were repeated in the patients who had recovered. All patients gave altered liver function tests results. Antipyrine half-life was significantly higher in females as compared to males. Among females, it was maximum in the pregnant group. Activities of drug metabolising enzymes were found to be significantly lower in liver biopsy material. The half-life of antipyrine showed a significant correlation with drug metabolising enzymes. After recovery patients showed a normal antipyrine half-life.