© 2016, Japanese Society of Anesthesiologists.Purpose: The hypothesis of our study is that during anesthesia, administration of 80 % oxygen concentration increases oxidative stress more than 40 % oxygen. Methods: Forty ASA I-II patients were included in a randomized, single-blind study. Expiratory tidal volumes (ETV) were measured before induction and after extubation. After ventilation with 0.8 FiO2 and intubation, mini-bronchoalveolar lavage (mini-BAL), arterial blood gas (ABG), and blood samples were taken. Patients were randomly assigned to receive 0.8 (group I) or 0.4 (group II) FiO2 during management. Before extubation, mini-BAL, ABG, blood samples were taken. PaO2/FiO2, lactate, malondialdehyde (MDA), protein carbonyl (PCO), superoxide dismutase (SOD), total sulfhydryl (T-SH), non-protein sulfhydryl (NPSH), and protein sulfhydryl (PSH) were measured. In both groups, mean arterial pressure and heart rate values were recorded with 30-min intervals. Results: ETV values were higher in group II after extubation. PaO2/FiO2 values were higher in group II after extubation compared to group I. In both groups, plasma PCO, SOD, and T-SH levels increased significantly before extubation, whereas the increase in MDA was not significant between groups. Plasma PCO, T-SH, and lactate levels were higher in group I, and plasma SOD, and PSH were higher in group I before extubation. In both groups, MDA, SOD, T-SH, and NPSH levels in mini-BAL increased significantly before extubation. Between-group comparisons, PCO, T-SH, PSH, and NPSH were significantly higher in the BAL samples of group II, and MDA levels were higher in group I. Conclusions: We found that 80 % FiO2 decreased ETV and PaO2/FiO2 and increased lactate levels and oxidative stress more, inhibiting antioxidant response compared to 40 % FiO2.