The purpose of this study was to investigate the effects of glucuronic acid (isolated from xylan) on antioxidative defense system in rat after aerobic exercise. The glucuronic acid was isolated from xylan. Sprague-Dawley male rats weighing 150$pm$10 g were randomly assigned to one normal group and three exercise training groups. Exercise training groups were classified to T (glucuronic acid free diet), TU (250 mg glucuronic acid/kg bw) and 2TU (500 mg glucuronic acid/kg bw) according to the level of glucuronic acid supplementation before exercise training. The experimental rats in exercise training groups (T, TU and 2TU) were exercised on glucuronic acid supplementation or rats in normal group (N) were confined in cage for 4 weeks. And rats were sacrificed with an overdose of pentobarbital injection just after running. Body weight, food intakes and food efficiency ratio (FER) were lower in the exercise training group than in the normal group. White gastrocnemius xanthine oxidase (XOD) activity in the T group was 85% greater than that of the normal group, whereas in the TU and 2TU groups it did not differ from the normal group. White gastrocnemius superoxide dismutase (SOD) activity in T group, that was decreased by 22% compared with that of N group, but those of TU and 2TU groups were increased by 38% and 42%, respectively, compared with that of T group. White gastrocnemius glutathione peroxidase (GSHpx) activity in T group, that was decreased by 42% compared with that of N group, but those of TU and 2TU groups were increased by 67% and 68%, respectively, compared with that of T group. Glutathione S-transferase (GST) activity of white gastrocnemius in N group was not significantly different from that in the T and TU groups, but 2TU group were increased by 12%. Contents of thiobarbituric acid reactive substance (TBARS) in T group was increased by 54%, compared with that of normal group but those of TU group and 2TU group were lower 44% and 36% than that of T group. In conclusion, the effects of glucuronic acids in exercise training rats would appear to reduce peroxidation of tissue as an antioxidative defense mechanism.