Effects of exercise on the formation of 8-hydroxydeoxyguanosine (8-OH-dG), a biomarker of oxidative DNA damage, and other purine metabolites such as hypoxanthine, xanthine and uric acid were examined. Venous blood and urine were collected from swimmers and distance runners before and after the usual training. The amount of 8-OH-dG obtained from nuclear DNA of lymphocytes decreased remarkably after intermittent swimming. The amount of nuclear 8-OH-dG also declined after distance running, but this difference is statistically not significant. After each exercise, plasma concentrations of hypoxanthine, xanthine and uric acid rose significantly. Urinary excretion of hypoxanthine increased, and xanthine and uric acid decreased after exercise. The 8-OH-dG-to-creatinine ratio in urine increased slightly after swimming or running. It is supposed that the repair of oxidative DNA damage is augmented by exercise. As far as we know, this is the first report concerning the effect of exercise on oxidative damage in nuclear DNA.