Trained yearlings had elevated muscle nitric oxide and total glutathione (GSH-T) as compared to mares at week 9. Trained mares and yearlings had increased erythrocyte glutathione peroxidase and GSH-T by week 9. Mares had higher lipid peroxidation than yearlings prior to training. After training, mares and yearlings had similar levels of oxidants and antioxidants.