Effects of chronic administration of ginseng extracts (30 or 150 mg/kg/day for 52 days, p.o.) to mice on the activities of DT-diaphorase and glutathione S-transferase (GST) in the liver and the brain were studied. The DT-diaphorase activity in the liver was increased over 2-fold at the dose of both 30 and 150 mg/kg/day, while there was no change in the activity of the enzyme in the brain. The GST activity in the liver was increased in a dose-dependent fashion up to 142% of the control value at the dose of 150 mg/kg/day. while there was no change in the activity of the enzyme in the brain. The ginseng-induced increase in the activities of these hepatic phase II drug-metabolizing enzymes which are involved in the detoxification of carcinogens, is suggested to underlie, at least in part, the anticarcinogenic activity of Panax ginseng.