The relationship between prolonged exercise, oxidative stress, and the protective capacity of the antioxidant defense system has been determined. Venous blood samples were removed from seven trained athletes before and up to 120 h after completion of a half-marathon for measurements of blood antioxidants, antioxidant enzymes, and indices of lipid peroxidation. Plasma creatine kinase (CK) activity, an index of muscle damage, increased (P < 0.05) to a maximum 24 h after the race but this was not accompanied by changes in conjugated dienes and thiobarbituric acid reactive substances (TEARS), which are indices of lipid peroxidation. An increase (P < 0.05) in plasma cholesterol concentration (4%) immediately after the race was similar to the change in plasma volume (6%). However, transient increases (P < 0.05) immediately postrace in the plasma concentrations of uric acid (24%), vitamin A (18%), and vitamin C (34%) were only partly accounted for by the fluid shifts. The immediate postrace increases in α- and γ-tocopherol did not attain statistical significance. Erythrocyte antioxidant enzyme activities were unaffected by the exercise but the α- and γ-tocopherol concentrations progressively increased (P < 0.001 and P < 0.05, respectively) up to 48 h postrace. Paradoxically, 24 h after the race erythrocyte susceptibility to in vitro peroxidation was markedly elevated (P < 0.01). This enhanced susceptibility to peroxidation was maintained even at 120 h postrace and did not correspond to changes in the age of the red cell population. A decrease (P < 0.001) in total erythrocyte glutathione immediately after the half-marathon was mainly due to a reduction in the reduced form (GSH). The results show that when trained athletes run a comparatively short distance sufficient to result in some degree of muscle damage but which is insufficient to cause elevations in plasma indices of lipid peroxidation, significant alterations in erythrocyte antioxidant status do occur.