BALB/c mice were exposed to an aerosol of the herbicide paraquat dichloride solution and were killed 6 h, 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. Inhaled paraquat solution produces pulmonary fibrosis that is morphologically similar to the lesions produced by systemic administration.