Oral administration of O,O,S-trimethyl phosphorothioate (OOS), an impurity in technical malathion, caused morphological changes in the bronchiolar epithelium of rat lungs. OOS-treated rat lungs had fewer but larger Clara (non-ciliated) cells than lungs from control rats given either corn oil or purified malathion. Moreover, lactate dehydrogenase (LDH) activity in bronchopulmonary lavage fluid was significantly higher in OOS than in control rats. We interpret these data to mean that OOS, and/or its metabolite(s) causes a lesion in the lung. Because of the widespread agricultural use of technical malathion, future work should address the significance of our findings and the possible toxic effect of OOS on lung tissue.