Plasma concentration of uric acid, total peroxyl radical-trapping antioxidative parameter (TRAP), blood lactate concentration and plasma activity of xanthine oxidase (XO) were measured in six Standardbreed trotters after six bouts of exercise with increasing intensity on two separate days three days apart. Blood samples were taken immediately, 5, 10, 15, 30 and 60 rain after each heat and 2, 4, and 6 hr after the last heat. Exercise caused an increase in TRAP and in the concentrations of lactate and uric acid. Plasma uric acid concentration increased exponentially with respect to time after the last heat performed at maximal speed, indicating a rapid increase in the rate of purine degradation. Plasma XO activity increased during exercise, but the intensity of exercise had only a minor effect on the level of XO activity. In conclusion, these data suggest that a threshold for the plasma accumulation of uric acid in terms of the intensity of exercise may exist and that XO may play a role in the formation of uric acid in horse plasma. Intense exercise causes an increase in the plasma antioxidant capacity that in the horse is mainly caused by the increase in the plasma uric acid concentration.