Morphometric and morphologic methods have been used to evaluate changes in rat lungs caused by the inhalation of a variety of oxidants. Exposure to 100% oxygen causes diffuse pulmonary injury and leads to death after 66–72 h of exposure. The primary insult leading to death in rats exposed to hyperoxia is injury to pulmonary capillary endothelium. Sublethal exposure to hyperoxia was found to cause diffuse injury to all major components of the alveolar septum and was associated with destruction of approximately 50% of the pulmonary capillary endothelial cells. A corresponding decrease in pulmonary capillary surface area and capillary lumen volume also occurred. Exposure to ozone and to nitrogen dioxide in low concentrations did not cause a diffuse injury throughout the alveolar region of the lung, but rather led predominantly to structural alterations in terminal bronchioles and in their adjacent alveoli. Morphometric evaluation of animals exposed to 0.25 ppm ozone and to 2 ppm NO2demonstrated quantitatively and qualitatively similar lesions. These lesions primarily involve Injury and remodelling of the alveolar epithelium. These changes in the alveolar epithelium were also associated with the recruitment of increased numbers of alveolar macrophages to the proximal alveolar region. The different types of lung injury caused by various oxidants are most likely to be related to differences in their reactivity with tissue components and to differences in concentration, distribution, and diffusion characteristics of the oxidant gases.