The pulmonary toxicity of the organophosphorus insecticide fenitrothion was evaluated following a single exposure of rats to the field formulation, at the site of an aerial spraying. Four groups of 40 Sprague-Dawley rats (including a control), set in wood enclosures, were placed under the aerial lines of the spraying aircraft. The degree of exposure was monitored at the ground level by air sampling and visual evidence of droplet activity deposition. Plasma pseudocholinesterase activity and pulmonary alveoli ultrastructure were used as indices to the fenitrothion exposure. Rat lungs were examined under light and electron microscopy at days 3, 7, 21, 60 and 180 after the exposure. Although a few signs of toxic lung injury were observed at days 3 and 7 there was no cholinergic crisis nor an effect on the pseudocholinesterase activity within 12 h in the exposed animals, when compared with controls. The alveolar toxic reaction was limited to small and discrete foci, and was entirely reversible within a period of 2 months. On a morphological basis it is thus concluded that a single field exposure to fenitrothion did not induce any permanent change in the alveolar area of the rat lung.