Effects of ginseng on in vivo antioxidant capacities with age were studied in rats. All rats were reared in the conventional system. Ginseng-treated rats were supplied with ginseng water extracts (25 mg/kg/day) continuously from 6 weeks of age to spontaneous death. None of the rats showed any discernible adverse effects of treatment with ginseng-containing water. There was no significant difference in body weight (BW) gains with age between treated and control groups. However, ginseng extracts did cause a decrease in the level of serum low density lipoprotein (LDL)-cholesterol, glucose, and thiobarbituric acid reactive substances (TBARS) in the treated rats. The activities of superoxide dismutase (SOD), catalase, and glutathione peroxidase in liver cytosol decreased with age in the control group. However, these enzyme activities were well maintained in the ginseng-treated rats and, especially, catalase and glutathione peroxidase activities were consistently higher than in control rats. The levels of total sulfhydryl group (T-SH) and glutathione reductase (GR) were unchanged, and glutathione-s-transferase (GST) activity gradually decreased with age in both groups. There were no differences in T-SH, GR, or GST between the control and treatment groups. These results indicate that long-term administration of ginseng retards age-related deterioration in some biochemical parameters such as cholesterol, glucose, and lactate dehydrogenase in serum and it has an enhancing effect on antioxidant capacity in the liver.