The response of erythrocyte antioxidant enzymes to a single bout of treadmill exercise was investigated in the Quarter Horse with or without dietary supplementation of Vitamin E (Vit E). Ten mares aged 5–11 yrs were randomly assigned to two dietary treatment groups. One group was fed a diet supplemented with 300 mg Vit E/kg as d, 1-tocopherol acetate, and the other group received a control diet. Treadmill conditioning consisted of an interval walk-trot regimen 5 days/wk for 7 weeks. At the end of the conditioning period the horses were subjected to a 12 min treadmill exercise test at 4.5 m/sec, 11% grade, while carrying 17 kg additional weight. Blood was collected from the jugular vein at rest, 2 min post-exercise and 30 min post-exercise. Plasma and erythrocytes were separated by centrifugation at 750 × g. Erythrocyte glutathione peroxidase (GPX), catalase (Cat) and superoxide dismutase (SOD) activities were not significantly altered by an acute exercise bout or during recovery. Vit E supplementation did not affect the antioxidant enzyme response to exercise except during recovery Vit E supplemented horses had higher GPX activity as compared to those receiving control diet. Plasma lactate and glucose levels were both significantly elevated by exercise bout and remained to be higher than resting levels during recovery. Blood hemoglobin concentration showed no alteration by either acute exercise bout or Vit E supplementation. These data indicate that dietary supplementation of Vit E did not alter the antioxidant status in equine erythrocytes. An exercise bout which increased blood lactate levels by 18-fold did not elicit a significant response of antioxidant enzymes. It is concluded that equine erythrocytes possess sifficient antioxidant capacity to cope with oxidative stress induced by exercise of this type.