Rats were injected with paraquat, 35 mg/kg, and killed 8 or 12 hours later. Electron microscopic studies of the lungs revealed degenerative changes in alveolar epithelium and alveolar capillary endothelium after 8 hours. After 12 hours, the alveolar lining layer was contaminated with cellular debris from disintegrating alveolar epithelium. This contamination is probably responsible for the inactivation of lung surfactant that can be demonstrated in early stages of experimental paraquat poisoning.