The effects of endurance training and acute exhaustive exercise on plasma levels of three superoxidedismutase (SOD) isoenzymes and the ability of superoxide generation in neutrophils were studied. Eighteen healthy male students, aged 17-22 years, who volunteered for this study, underwent three months of endurance training in swimming or running. Before and after the training course, they performed acute exercise and blood samples were collected before and after this exercise. The endurance training significantly increased maximal oxygen uptake (VO2max) in all subjects. Neither the endurance training nor the acute exercise affected the plasma CuZn-SOD level. Acute exercise after the training, but not before the training, increased both the plasma Mn-SOD and extracellular SOD (EC-SOD) levels by 33.6 and 33.5%, respectively. The training decreased the EC-SOD level at rest by 22.2%. Acute exercise after the training, but not before the training, increased the plasma lipid peroxide level, suggesting higher oxidative stress in trained subjects during exhaustive exercise. The ability of neutrophils to generate superoxide was increased by the acute exercise, but induction of the superoxide was suppressed after training. These results indicate that EC-SOD levels were changed in a different manner from the CuZn-SOD and Mn-SOD: it was decreased by training but was increased by acute exercise, suggesting that endurance training increases the reserve of EC-SOD in tissues. The results also suggest the possibility of plasma EC-SOD assay as a new index of endurance training.