Oxidative stress is an imbalance of the oxidant to antioxidant ratio in the body. In the following presentation, I will highlight studies from my laboratory along with other pertinent studies of oxidative stress in exercising horses. An increase in oxidative stress and changes in anti-oxidant status has been shown during endurance, intense exercise, and eventing competition in horses. Antioxidants are vitamins, minerals, and protein that must be synthesized in the body or obtained from the diet. Therefore, exercise level and diet are both factors that play a role in influencing the oxidative stress and antioxidant status of the equine athlete. Along with exercise intensity, duration and diet, age and conditioning program also have affected oxidative stress in the horse. The “free radical theory of aging” states that long-term effects of the degenerative changes associated with aging may create oxidative stress. However, in old horses (22 ± 2 yr), the amount of lipid peroxidation and blood antioxidant concentrations were similar to those found in mature but younger (12 ± 2 yr) horses. Same as older horses that may require dietary intervention to help combat oxidative stress so might young growing exercising horses. Studies have shown that yearlings (18 ± 2.4 mo) did not begin their exercise training with higher levels of oxidative stress in muscle or blood than mature mares (13 ± 2.1 yr). Prior to exercise conditioning, yearlings had lower lipid peroxidation and higher antioxidants than mature mares. Conditioning reduced oxidative stress and improved antioxidant status in mares, while few effects were seen in yearlings. This suggests that age alone was the biggest defense against oxidative stress after exercise. Other studies during competition (endurance, jumping, eventing, and racing) have investigated the influence on oxidative stress with varying results. These results will be expanded upon during the presentation. Even though there have been many studies examining the levels of lipid peroxidation, antioxidant status and other related metabolites in the horse during exercise, we still have a long way to go before we fully understand the large variation in results both with and without antioxidant supplementation.