The objective of this study was to determine whether running a 24-h race would cause oxidative damage and changes in the blood antioxidant defense capacity in endurance-trained athletes. Fourteen male amateur runners (mean age 43.0±10.8 y, body weight 64.3±7.2 kg height 171±5 cm, weekly covered distance 81±43 km, training history 8±9 y) who participated in a 24-hr ultra-marathon and volunteered to give blood samples during the race were enrolled for this study. Blood samples were taken before the run, after completing the marathon distance (42.217 km), after 12 h and at the conclusion of the race.The capacity of erythrocyte antioxidant defense system was evaluated by measuring the activities of superoxide dismutase (SOD), glutathione peroxidase (GPX), catalase (CAT), glutathione reductase (GR), concentrations of non-enzymatic antioxidants (uric acid and glutathione-GSH), and selected biomarkers of oxidative stress (i.e., plasma level of malondialdehyde (MDA) and plasma antioxidant capacity by FRAP ("ferric-reducing ability of plasma")). Moreover, in order to elucidate between-group differences in the total capacity of the blood antioxidant defense system, an index of antioxidant potential (POTAOX) was calculated as a sum of standardized values of activities of antioxidant enzymes (SOD, CAT, GPX, GR) and non-enzymatic antioxidants (uric acid, GSH).A progressive decline was observed in activities of SOD and CAT with the distance covered during the race, while the opposite trend was found in activities of GPX and GR that tended to increase. A significant decrease was recorded in GSH content after completing the marathon distance, which tended toward slightly higher values, without reaching the baseline level, at the finish of the race. Plasma concentration of uric acid (UA) was not significantly affected, except for the value recorded after 12 h of running that was significantly (p<0.05) lower, while both markers of oxidative stress (FRAP and MDA) increased significantly after completing the marathon distance. Comparison of the calculated values of the POTAOX index recorded pre-race and throughout the competition implies that the most drastic decline in the total antioxidant capacity occurred at mid-race (i.e. after 12 h of running).