Background The intensive exercise increases circulating radical oxygen specie levels in humans. Horses are athletic animals that can be trained to execute performances well established by International Equestrian Federations. They are useful models to study their physical performances and related structural and metabolic derangements. Physical exercise increases both tissue needs for oxygen and cellular respiration and causes an overproduction of free radicals. When free radical generation exceeds the cell’s antioxidant capacity tissue-damage develops due to oxidative stress. Therefore, it appears important to increase the scavenger ability of the tissues. Controlled training and dietary supplements may provide ways of doing this. Materials and methods The animals were ten Arabian strain horses, 6-8 years old, trained for Cross country and six horses at rest. Blood samples are tested for Glutathione Pero-Xidase (GPX) and chemical parameters both before and after standard exercise. As a model, we used 3-year-old racehorses (Equus caballus) which underwent a series of different physical exercise trials before and after 70 days of daily training and dietary supplement (vitamin E). Results Glutathione peroxidase activity determined in exercised animals and radical oxygen species at rest and pre and post-exercise significantly decrease after exercise with P: 0.008 by Student-T. The vitamin addition (50 μM) to red blood cell membranes brings the activity to reference level. The above treatments were able to increase the glutathione peroxidase activity in lymphocytes. Moreover, they were also able to decrease vitamin E consumption following the physical exercise trials. Discussion Erythrocyte membranes are damaged by standard exercise and GPX a is almost restored to reference and pre-exercise levels by in vitro treatment with vitamin E. The results obtained indicated that the training and diet supplement (vitamin E) we used were able to significantly increase horse antioxidant defences in both the extracellular fluids and blood cells of our horses, thus decreasing peroxidative phenomena following physical exercise.