Purpose of this study was to investigate the effect of single bouts of exercise at 3 different intensities on the redox state of human serum albumin (HSA) and on carbonyl groups on protein (CP) concentrations in plasma. Trained men (n = 44, VO₂max: 55 ± 5 mL x kg^-1 x min^-1, non-smokers, 34 ± 5 years of age) from a homogenous population, volunteers from a police special forces unit were randomly assigned to perform on a cycle ergometer either at 70% (n = 14), 75% (n = 14), or 80% (n = 16) of VO₂max for 40 minutes. Blood was collected before exercise (BE), immediately after exercise test (IE), 30 minutes (30M) and 30 hours (30H) after each test. The reduced fraction of HSA, human mercapt albumin (HMA), decreased at all 3 exercise intensities IE and 30M, returning to pre-exercise values by 30H (P < 0.05). HMA was primarily oxidized to its reversible fraction human nonmercapt albumin 1 (HNA1). CP concentrations increased at 75% of VO₂max IE and 30M with a tendency (P < 0.1) and at 80% VO₂max IE and 30M significantly, returning to pre-exercise concentrations by 30H (P < 0.01). These results indicate that the HSA redox system in plasma is activated after a single bout of cycle ergometer exercise at 70% VO₂max and 40 minutes duration. Extent of the HSA modification increased with exercise intensity. Oxidative protein damage, as indicated by CP, was only significantly increased at 80% VO₂max intensity in this homogenous cohort of trained men.