To evaluate the potential risk of pulmonary damage due to aerial spraying of the insecticide fenitrothion, rat lungs were examined under light and electron microscopy at 3, 7, 21, and 60 days after exposure. Rats were exposed by a "nose-only" apparatus for 1 hr to 2 or 500 mg/m3 of aerosolized fenitrothion (15%) mixed with solvent Cyclosol 63 (35%) and diluent oil 585 (50%). Aerosol size particles were monitored by a light scattering apparatus. Only minor modifications of lung alveolar tissues were observed after exposure to the high concentration. At 3 days, discrete foci of mild inflammation were detected, including interstitial edema, cellular infiltration, and increased number of alveolar macrophages. At 7 days, signs of irritation were diminished while at 21 and 60 days alveolar tissues were essentially normal. Exposure to lower concentration induced more limited changes at 3 days; no modifications were seen at later periods. It is concluded that a single exposure to this fenitrothion mixture at 500 mg/m3 presents no serious hazard of pulmonary toxicity.