The level of oxidative stress markers and total antioxidant capacity in the blood of Ukrainian warmblood horses and Hanoverian horses at the rest and after training was studied. There were significant changes in the thiobarbituric acid reactive substrates (TBARS) content and lipid hydrtoperoxides level after training both in the erythrocytes and plasma of Ukrainian warmblood horses. A significant difference in lipid peroxidation in the blood of horses of both breeds under exercise influence was occurred. The increase of total antioxidant capacity in the erythrocytes and plasma in Hanoverian horses after training was observed. Level of aldehyde and ketonic derivatives of oxidatively modified proteins were significantly differed between horses of both breeds. There were significant decreases in the values of the aldehyde derivatives of protein oxidation in the plasma after training in Ukrainian warmblood horses. The level of oxidative stress markers in the blood of sport horses can be sensitive and informative parameters for the assessment of horse’s performance.