The purpose of this study was to investigate the intensity of exercise load which increases urinary 8-hydroxydeoxyguanosine (8-OHdG) excretion and the effect of antioxidant supplementation on urinary 8-OHdG excretion after a single bout of exercise. The subjects included 6 healthy males with the following characteristics : age ; 24.0±1.1 years, height ; 174.0±8.5 cm, weight ; 71.5±15.4 kg, BMI ; 23.2±3.4 kg/m². The urinary concentration of 8-OHdG was measured by the two-column-switching high-performance liquid chromatography (HPLC) method. After 7 days of supplementation, the time course of the urinary 8-OHdG excretion was observed following treadmill running at 70% VO₂max.Significant increases in the urinary 8-OHdG excretion were detected at 2 h (p<0.01) and 4 h (p<0.05) after exercise.After 7 days of supplementation, a significant increase in the urinary 8-OHdG excretion was detected 1 h after exercise (p<0.05); however, it returned to the initial level 2 h after exercise. Therefore, oxidative DNA damage induced by a single bout of exercise was diminished by antioxidant supplementation.