Selected estimates of the lipid peroxidative capacity were assayed in the red and white skeletal muscles of control and endurance-trained mice. Endurance training decreased the lipid peroxidation rate in vitro in both muscle types. The concentration of lipids susceptible to Fe2+-induced lipid peroxidation was greater in the red than in the white skeletal muscle and increased after endurance training in the red muscle. Endurance training, however, decreased highly significantly the sensitivity of red muscle to in vitro stimulated lipid peroxidation. The activity of catalase and the concentration of vitamin E were considerably higher in the red muscle, whereas the activity of glutathione peroxidase was slightly higher in the white muscle. Endurance training caused no changes in these antioxidants. Endurance training increased the concentrations of reduced and total non-protein glutathione in the red skeletal muscle but not in the white muscle. The total sulfhydryl group contents were unaffected. Our results suggest that endurance training may increase the resistance of skeletal muscle to injuries caused by lipid peroxidation.