Performance of strenuous physical activity can increase oxygen consumption by 10- to 15-fold over rest to meet energy demands. The resulting elevated oxygen consumption produces an "oxidative stress" that leads to the generation of free radicals and lipid peroxidation. A defense system of free radical scavengers minimizes these dangerous radicals. Indirect measurements of free radicals generated during exercise include assessing products of lipid peroxidation that appear in the blood (e.g., malondialdehyde and conjugated dienes) or expired in the breath (pentane). Changes in antioxidant scavengers and associated enzymes (e.g., glutathione, tocopherol, glutathione peroxidase) also provide clues about demands on the defense system. Physical training has been shown to result in an augmented antioxidant system and a reduction in lipid peroxidation. Supplementation with antioxidants appears to reduce lipid peroxidation but has not been shown to enhance exercise performance. The "weekend athlete" may not have the augmented antioxidant defense system produced through continued training. This may make them more susceptible to oxidative stress. Whether athletes or recreational exercisers should take antioxidant supplements remains controversial. However, it is important that those who exercise regularly or occasionally ingest foods rich in antioxidants.