The significance of free radical chemistry within the exercise and post-exercise milieu is not yet well understood. It is yet to be determined whether adequate biochemical defense mechanisms exist to protect the organism from oxygen-centered radicals generated by exercise. Rats trained at 70% VO2peak for 6 wk were compared with controls after an exhaustive run. Post-exhaustion urinary malondialdehyde, gastrocnemius loosely bound iron, and susceptibility to oxidant stress were assessed. Exhaustive exercise resulted in a significant (P < 0.05) increase in urinary malondialdehyde, tissue loosely bound iron, and susceptibility to oxidative stress in both control and trained rats. The untrained group's tissue iron and susceptibility to oxidative stress were both significantly greater than trained rats. Electrical stimulation of perfused hindquarters of untrained and trained rats resulted in a significant increase of malondialdehyde into the perfusate. Trained rats cleared the malondialdehyde from the perfusate more rapidly than did the untrained.