It is well known that physical training permits an animal to respond successfully to exercise loads of various types, intensities, and durations. Furthermore, the trained animal can sustain the activity for a long period before the fatigue becomes limiting. The effects of physical training on the antioxidant defenses of tissues and on their susceptibility to damage induced by exhaustive exercise have been investigated. Therefore, untrained rats were sacrificed either at rest or immediately after swimming to exhaustion. Rats trained to swim for 10 weeks were also sacrificed, 48 hr after the last exercise, either at rest or after exhaustive swimming. Homogenates of liver, heart, and muscle were used for biochemical determinations. Mitochondrial and sarcoplasmic (SR) or endoplasmic (ER) reticulum integrity was assessed with measurements of respiratory control index and latency of alkaline phosphatase activity. Lipid peroxidation was measured by determination of malondialdehyde and hydroperoxides. Additionally, the effect of training on the antioxidant protection systems of tissues was examined by determining the glutathione peroxidase and glutathione reductase activity and the overall antioxidant capacity. Mitochondrial, SR, and ER integrity and lipid peroxidation were similar in trained and untrained at rest animals, whereas the glutathione peroxidase and glutathione reductase activity and the overall antioxidant capacity of tissues were greater in trained animals. The exhaustive exercise gave rise to tissue damage irrespective of the trained state, as documented by similar loss of SR and ER integrity, and by increase in lipid peroxidation found in exhausted trained and untrained rats. Because exercise endurance capacity was greatly increased by training, our results suggest that free radical-induced damage in muscle could be one of the factors terminating muscle effort. In effect, the greater antioxidant level should allow trained muscle to withstand oxidative processes more effectively, thus lengthening the time required so that the cell function is sufficiently damaged as to make further exercise impossible.