Physical exercise increases a production of reactive oxygen species (ROS) in equine organism. ROS causes a damage of cell membranes of tissues by lipid peroxidation. Muscle tissue is more susceptible to lipoperoxidation in comparsion with other body tissues, because it contains less antioxidants, which could protect it against over-production of ROS. The antioxidant defense system consists of enzymatic and non-enzymatic antioxidants. Animals do not have such a high concentration of non-enzymatic antioxidants in comparison with human. Therefore, mainly horses enrolled in top-level sport requires intake of antioxidants in the diet. One of the suitable supplements of antioxidants in the diet for thoroughbred horses in full training appears to be an addition of evening primrose oil (Oenothera biennis) with higher content of fatty acid such as ?-linolenic acid, ?-linolenic acid, linoleic acid and oleic acid. The aim of the study was to confirm the improvement of the antioxidant capacity of the organism of thoroughbred horses in maximal exercise after the supplementation of evening primrose oil. 16-week experiment was conducted on 10 clinically healthy thoroughbred horses. Each of these horses was in normal training program. The first 8 weeks of the experiment, horses were fed by a diet without primrose oil (first 3 blood tests) and another 8 weeks the same diet was supplemented with 150 ml of evening primrose oil (next 3 blood tests). The monitored indicators of antioxidants levels were the total antioxidant activity (TAS) and uric acid (UA). The rate of lipid peroxidation was measured by thiobarbituric acid reactive substances (TBARS) and it was accompanied by monitoring of changes in muscle enzyme activities of aspartate aminotransferase (AST) and creatine kinase (CK). We also monitored the influence on the energy and fatty metabolism by selected metabolites such as glucose, triacylglycerols (TAG) and the hormone insulin. Biochemical analyzes were performed by commercial kits. The were statistically evaluated samples with addition of EPO (4th, 5th, 6th sampling) and 3rd sampling was determinated as a control (means 0 day addition of EPO). Previous samples were not included in the statistical monitoring. It was proved that the intake of primrose oil in amount of 150 ml significantly decreased the values of TBARS, insulin and increased levels of blood glucose and TAG. Study did not prove a statistically significant effect on levels of uric acid , TAS and muscle enzymes AST and CK. It can be concluded that evening primrose oil supplement had a positive influence on the reduction of oxidative stress of the organism and on the energy metabolism of race horses during exercise and its usage can be recommended as an effective supplement of diet for improving the health status of horses during exercise.