Background. Red blood cells (RBCs) are crucial for tissue oxygen delivery during exercise. Strenuous exercise can affect RBC membrane structure, probably due to mechanical and chemical stress, which ultimately may reduce tissue oxygen supply, and impair exercise performance. The objective of this study was to investigate oxidative stress biomarkers in RBCs during modern pentathlon. Methods. Six athletes from the Brazilian Modern Pentathlon national team (4 men, 27 ± 4 yr, 1.8 ± 0.1 m, 70.5 ± 8.7 kg) participated in a simulated competition of modern pentathlon, which lasted about 8h. Blood samples were collected at 9 different times: T1, rest; T2, post-fencing; T3/T4, pre/post-200 m swimming; T5/T6, pre/post-horse riding; T7/T8, pre/post-combined event (3 km run interspersed by shooting); T9, after 2 h-recovery. RBCs were isolated by centrifugation for the following experiments: osmotic fragility test; oxidative damage (lipid peroxidation and protein carbonylation); antioxidant enzymes (catalase and glutathione peroxidase, GPx) activities. The procedures conformed to the Declaration of Helsinki and written informed consent was obtained form all subjects. Data were compared against resting values with a paired t-test. Significance level was set at 5%. Results. All results are shown in Table 1. Osmotic fragility was significantly reduced after swimming (T4) and horse riding (T6) compared to rest (T1). A massive increase in lipid oxidative damage was observed after horse riding, which remained altered up to 2 hours after the end of competition. Protein oxidative damage was observed after horse riding (T6), and returned to basal levels thereafter. After horse riding (T6), a reduction in GPx activity was also observed, which remained depressed until recovery (T9). There were no significant variations in catalase activity throughout the competition, only a slight reduction before horse riding (T5). Conclusion. Modern pentathlon induces damage to RBC lipid structures, especially after horse riding. This may result from a depletion of the antioxidant enzyme GPx, whose main biochemical function is to reduce lipid hydroperoxides to their corresponding alcohols. Future studies should investigate whether the observed damage is capable of inducing changes in RBC membrane structure, resulting in increased rigidity and impairment in tissue oxygen supply.