The present investigation was designed to evaluate the acute effect of aerobic exercise on oxidative stress and the flow properties of the blood. Fourteen clinically healthy subjects (7 men and 7 women aged 56+/-19 yr) underwent maximal treadmill exercise with blood samples drawn prior to and immediately after exercise. Post-exercise significant increases were observed in plasma lipid hydroperoxides from 6.5+/-2.0 microM to 7.9+/-1.9 microM (p<0.0001) and the relative concentration of plasma fluorescent products associated with red cell peroxidation from 138+/-28 RF to 220+/-92 RF (p<0.005). After exercise there was a rise in the hematocrit from 41.4+/-3.7% to 44.4+/-4.1% (p<0.0001), increases in whole blood viscosity at shear rates of 22.5/sec to 450/sec (p<0.0005), an increase in plasma viscosity from 1.27+/-0.12 cP to 1.36+/-0.11 cP (p<0.01), an increase in red cell rigidity from 2.44+/-0.48 cP to 2.62+/-0.42 cP (p<0.001) and a decrease in erythrocyte sedimentation rate from 26.9+/-18.6 mm/h to 22.5+/-15.9 mm/h (p<0.01). The findings suggest that acute aerobic exercise induces oxidative damage to red blood cells and adversely affects rheological properties of the peripheral blood.