BALB/c mice were exposed to an aerosol of paraquat dichloride solution and were killed six hours, 1, 3, 7, 14 and 28 days later. Initial necrosis and sloughing of the bronchiolar and alveolar epithelia with intact endothelium were followed by type 2 pneumocyte hyperplasia, fibroblast proliferation, and increased synthesis of collagen. These results indicate that inhaled paraquat solution produces pulmonary fibrosis that is morphologically similar to the lesions produced by systemic administration.