We investigated the acute effects of long-distance running on oxidation of lipids and antioxidant functions in LDL and serum. Eight trained male runners who participated in a 31-km run and 22 male keep-fit runners who participated in a marathon run were enrolled into the study. Venous blood samples were taken before and immediately after the exercise. There were no changes in LDL diene conjugation (LDL-DC) or LDL antioxidant potential (LDL-TRAP) during the exercises. Serum (S-) TRAP and S-alpha-tocopherol rose during the 31-km run (by 22%, p = .0005, and by 29%, p = .011, respectively), and during the marathon (by 16%, p = .0014, and by 7%, p = .031, respectively). S-DC rose during the 31-km run (by 9%, p = .0026), but not during the marathon (p = .14). Preexercise and postexercise S-alpha-tocopherol correlated positively with pre and postexercise S-TRAP in the marathon run (r = .473, 95% CI 0.064 to 0.746, and r = .524, 95% CI 0.131 to 0.774, respectively). Thus, the paradoxical exercise-associated increase in S-TRAP is, at least in part, explained by a simultaneous rise in S-alpha-tocopherol concentration. However, acute exercise does not change LDL-DC or LDL-TRAP concentrations.