We have determined the plasma 8-hydroxy-deoxyguanosine (8-OHdG) levels during endurance exercise. Eight untrained male subjects completed a 10-km run. Plasma 8-OHdG, total coenzyme Q10 (CoQ10), ubiquinol, and thiobarbituric acid (TBA) concentrations were measured after exercise. Plasma 8-OHdG concentrations significantly decreased immediately after (0.21±0.13 ng/ml, p<0.01) and 1 hr after (0.23±0.09 ng/ml, p<0.05) the run compared to the resting values (0.36±0.09 ng/ml). Both plasma CoQ10 and ubiquinol concentrations significantly increased (p<0.05) immediately after the run compared to the resting values. On the other hand, plasma TBA concentrations did not change significantly at any point after the run. These results suggest that, during and after submaximal endurance exercise in this study, an augmented antioxidant defence system such as CoQ10 might thus play a role in the decrease of 8-OHdG in the plasma, and that exercise might stimulate the repair of oxidative damage to DNA.