We investigated the effect of a single bout of intensive exercise on the excretion of 8-hydroxy-deoxyguanosine in the 24 h urine from healthy non-smokers. We studied three exercise tests in Experiment 1; which consisted of incremental exercise to exhaustion on a treadmill in eleven male long distance runners. Experiment 2; which comprised incremental exercise until reaching exhaustion on a bicycle ergometer in sis male untrained subjects. Experiment 3; which consisted of a 20 km run by eleven male long distance runners. No differences in the urinary 8-hydroxy-deoxyguanosine excretion were observed from days 1 or 3 after each respective exercise regimen. However, significant increases in the plasma creatine kinase activity were observed at 24 h or 48 h after exercise, except for Experiment 2. Our results thus suggest that the oxidative stress during a single bout of intensive exercise does not result in an accumulation of oxidative DNA damage.