Reduction products of oxygen, including superoxide anion ( O 2 · ¯ ) , hydrogen peroxide (H2O2), and hydroxyl radical (•OH), have been implicated in the pathogenesis of oxygen toxicity, 1 Gerschman R Biological effects of oxygen. in: Dickens F Neil E Oxygen in the animal organism. MacMillan, New York1964: 475-494 Crossref Google Scholar , 2 Clark JM Lambersten CJ Pulmonary oxygen toxicity: A review. Pharmacol Rev. 1971; 23: 37-133 PubMed Google Scholar , 3 Deneke SM Fanburg BL Normobaric oxygen toxicity of the lung. N Engl J Med. 1971; 303: 76-86 Crossref Scopus (306) Google Scholar drug-induced lung injury, 4 Boyd MR Metabolic activation of pulmonary toxins. in: Witshi H Nettesheim P Mechanisms in respiratory toxicity. CRC Press, Boca Raton, FL1982: 85-112 Google Scholar , 5 Sasame HA Boyd MR Superoxide and hydrogen peroxide production and NADPH oxidation stimulated by nitrofurantoin in lung microsomes: Possible implications for toxicity. Life Sciences. 1979; 24: 1091-1096 Crossref PubMed Scopus (61) Google Scholar and experimental and clinical ARDS. 6 Repine JE Bowman CM Tate RM Neutrophils and lung edema. State of the art. Am Rev Respir Dis. 1982; 81S: 47S-50S Google Scholar , 7 Rinaldo JE Rogers RM Adult respiratory distress syndrome. Changing concepts of lung injury and repair. N Engl J Med. 1982; 306: 900-909 Crossref PubMed Scopus (372) Google Scholar These toxic O2 species are referred to informally as O2 radicals, and can be generated in the lung from many sources including release from neutrophils and macrophages, 8 Goldstein IM Roos D Kaplan HB Weissman G Complement and immunoglobulins stimulate superoxide production by human leukocytes independently of phagocytosis. J Clin Invest. 1975; 56: 1155-1163 Crossref PubMed Scopus (503) Google Scholar metabolism of certain drugs, 9 Kappus H Sies H Toxic drug effects associated with oxygen metabolism: Redox cycling and lipid peroxidation. Experientia. 1981; 37: 1233-1241 Crossref PubMed Scopus (538) Google Scholar and inhalation of oxidant gases. Since O2 radicals are known to interact with many substances, including lipids and proteins that are vital to cell structure and function, we became interested in the hypothesis that O2 radicals might affect the permeability of the alveolar-capillary membrane and thereby contribute to the pathogenesis of the permeability pulmonary edema characteristic of the above disorders. In this study, O2radicals are generated by either xanthine oxidase or glucose oxidase and their effects on isolated lungs are determined.