Oxygen is indispensable for aerobic respiration. However, the effects of hyperoxia on the lungs are poorly defined. The aim of the present study was to determine the effects of different oxygen concentrations on rat lungs. Rats (n = 6 per group) were exposed to hyperoxia for 90 minutes at 3 different concentrations: 50% (H50%), 75% (H75%), or 100% (H100%). Bronchoalveolar lavage (BAL) was performed and the right lungs were removed for histological analyses. The BAL samples were assayed for lipid peroxidation and antioxidant status using biochemical methods. Hyperoxia induced influxes of macrophages (1.8- to 2.3-fold) and neutrophils (7.0- to 10.2-fold) into the lungs compared to the control group (exposed to normoxia; n = 6). Histological analyses of the hyperoxic groups showed hemorrhagic areas and septal edema. A significant increase (2.2-fold) in lipid peroxidation was observed in the H100% group compared to the control group (P <.05). Glutathione peroxidase and superoxide dismutase activities were reduced to approximately 20% and 40% of the control values, respectively, in all 3 hyperoxic groups, and catalase activity was reduced in both the H75% (-0.6-fold) and H100% (-0.7-fold) groups. These results indicate a harmful effect of hyperoxia on the rat lung, with evidence of oxidant/antioxidant imbalance and histological damage.