Purpose and Methods: Indices of antioxidant status, membrane permeability, and lipid peroxidation were investigated in venous blood immediately before and after a simulated half-marathon ran. In serum, these included the ability to scavenge free radicals (total antioxidant capacity, TAG), the concentration of uric acid (UA), and the activities of creatine kinase (CK) and beta-glucuronidase (beta G). The plasma concentration of malondialdehyde (MDA) was used as a marker of lipid peroxidation. Data were analyzed with paired t-tests. After a standardized warm-up, 17 trained male runners (mean +/- SD, age 31 +/- 4 yr, peak (V) over dotO(2) 63.2 +/- 4.8 mL.kg(-1).min(-1)) each completed a self-paced half-marathon run, on a motorized treadmill. Average exercise intensity was 77.1 +/- 1.0% peak (V) over dotO(2), with a performance time of 87.1 +/- 7.0 min. Results: After exercise, elevations were observed in MDA from 1.48 +/- 0.39 mmol.L-1 to 1.65 +/- 0.32 mmol.L-1 (P < 0.05), TAC from 475 +/- 84 to 564 +/- 113 mmol Trolox Eq.L-1 (P < 0.0001), UA from 268 +/- 45 to 312 +/- 51 mmol.L-1 (P < 0.001), serum cortisol concentration from 339 +/- 95 to 557 +/- 157 nmol.L-1 (P < 0.01), CK from 98 +/- 67 to 133 +/- 89 IU.L-1 (P < 0.0001), and beta G from 15.39 +/- 5.34 to 17.05 +/- 5.7 Sigma Units.mL(-1) (P < 0.001). Conclusions: The rise in TAC did not prevent exercise-induced lipid peroxidation and muscle damage as both MDA and CK were elevated after exercise. This may indicate inadequacies in the antioxidant defense system during the half marathon run.