Acute exercise may induce free-radical production in mitochondria during basal metabolism of aerobic cells. Ascorbic acid is a strong antioxidant agent, whereas myoglobin is known to act as an oxygen reservoir. The purpose of this study was to investigate the effects of acute exercise on malondialdehyde levels in gastrocnemius, vastus medialis, and triceps brachi muscles and erythrocytes. In addition, we investigated the ascorbic acid levels and serum myoglobin concentrations in rats, following acute swimming exercise. We found that the levels of muscle malondialdehyde and serum myoglobin increased and the levels of plasma ascorbic acid decreased, in proportion to the duration of exercise; however, the levels of erythrocyte malondialdehyde did riot change.