Electron microscopic studies of the rat lung were carried out at various intervals after subcutaneous injection of paraquat. Within 12–18 hours after paraquat injection, capillary engorgement and interstitial oedema were noted in alveolar walls. After 24 hours there was, in addition, mitochondrial degeneration and cytoplasmatic vacuolization in granular pneumocytes, membraneous pneumocytes, and capillary endothelial cells. Desquamation of necrotic alveolar epithelial cells was a characteristic feature 48 hours after injection of paraquat. Many of these desquamated cells were identified as granular pneumocytes from their preserved lamellar inclusions. The severe ultrastructural lesions of the alveolar epithelium may serve to explain the derangement of pulmonary surfactant which has been observed in paraquat poisoning under similar experimental condition