Pulmonary oxygen injury is classified by the development of tissue and alveolar edema, surfactant dysfunction, lung inflammation, and decreased pulmonary compliance. In neonates prolonged oxygen therapy is associated with the development of bronchopulmonary dysplasia. Recombinant DNA technology makes it possible to experimentally explore the role of specific proteins in the development of pulmonary oxygen injury. However, in vivo experiments require sensitive ways of identifying pulmonary oxygen injury early in its development. We therefore compared the sensitivities of several experimental assays used to assess pulmonary injury. We found that changes in pulmonary compliance were the most sensitive and showed significant differences after 72 hr of exposure to normobaric hyperoxia (FiO2 = 0.95), which correlated with a small change in the histology of the mice lungs. The concentration of protein in the bronchoalveolar lavage fluid was less sensitive and did not differ significantly until after 96 hr of exposure. The survival in hyperoxia also did not worsen until after 96 hr. The lung wet/ dry weight ratios was the least sensitive assay and did not increase until after 5 days of exposure to normobaric hyperoxia. We conclude that a decrease in pulmonary compliance is an early indicator of pulmonary oxygen injury and may be a better way to study the mechanisms and mediators of pulmonary oxygen injury.