The present study was carried out to investigate the effect of exercise-induced oxidative stress on methemoglobin (Met-Hb), serum free hemoglobin (Hb) and lipoperoxide concentrations before and after 3 km run at differential running speed.Met-Hb concentration in runners was higher than that in untrained subjects at rest, whereas RBC (red blood cell) count and Hb concentration were lower in runners than those in untrained subjects. No significant changes of serum free Hb and lipoperoxide concentrations occurred. Serum free Hb concentration was found to decrease at low running speed (40%V4 O2max), and to increase at high running speed (80%V4 O2max), although Met-Hb concentration did not change. Similarly, blood lactate concentration decreased at low running speed and increased at high running speed. These results suggest that hemolysis to the red blood cells during running is caused by footstrike and fragility of RBC membrane, and partly oxidative stress associated with running at more than 80%V4 O2max.