While antioxidant supplementation apparently decreases exercise-induced oxidative stress, there is also a risk of attenuating the physiological response of tissues to exercise and blunting the adaptations. In this study we examined the protective role of natural thiol antioxidant, a-lipoic acid (LA) supplementation at rest and during recovery after an acute exercise in plasma and in skeletal muscle of horse. Six standardbred trotters were examined on the treadmill and exercised 75 min at individually defined aerobic level. They were supplemented orally with LA (25 mg kg^-1 day^-1) for five weeks without any additional vitamins. Using electro paramagnetic resonance assay, we showed that strenuous aerobic exercise increases significantly free radical formation in gluteus medius muscle during control period but not after LA-supplementation. LA-supplementation decreased the amount of post-exercise lipid hydroperoxides in plasma and exercise-induced increase in malondialdehyde in plasma and muscle. There was no difference in muscle protein carbonyl levels; a marker of protein oxidation. Furthermore, LA increased the oxidative capacity of the muscle measured by citrate synthase activity and lowered the blood lactate concentration during exercise. Our results suggest that LA-supplementation appears to decrease exercise-induced oxidative stress at lipid phase without impairing the athletic capacity of the muscle in horse.