Exposure to hyperoxia results in endothelial necrosis followed by type II cell proliferation. This suggests that type II cells are resistant to hyperoxia. Oxygen-induced lung injury may result from an overproduction of oxygen metabolites normally scavenged by antioxidants such as superoxide dismutase (SOD), glutathione peroxidase, catalase and reduced glutathione (GSH). Therefore, resistance of type II cells to hyperoxia may be linked to high antioxidant activities. To test this hypothesis we compared in vitro the effects of a 24 h exposure period to 95% O2 on cultured type II cells, lung fibroblasts and alveolar macrophages isolated from rats. We show that type II cells, when compared with other cell types, are highly sensitive to hyperoxia as shown by increased lactate dehydrogenase (LDH) release, decreased deoxyribose nucleic acid (DNA) and protein content of Petri dishes and decreased thymidine incorporation into DNA. Synthesis of dipalmitoylphosphatidylcholine was also significantly reduced. Antioxidant enzyme activities as well as glutathione content were not higher in type II cells than in other cell types. However, hyperoxia results in a decreased SOD activity and glutathione content in type II cells which was not observed in fibroblasts. We conclude that adaptative changes in SOD and glutathione metabolism could be important defence mechanisms in cells exposed to hyperoxia.