Ethanol-treated and ethanol-nontreated rats were exposed to 0, 500, 1,000, 2,000, 4,000 and 8,000 ppm of trichloroethylene for 2 h in order to evaluate the metabolic (metabolic rate of styrene) and morphologic changes in the lung. The exposure resulted in dose-dependent decrease of styrene metabolism. By MPA (Maclura pomifera agglutinin) stain it was revealed by light microscopy that trichloroethylene caused a highly selective damage to nonciliated bronchiolar epithelial (Clara) cells, which were characterized by flattened cells. The ratios of the length of apical surface (a) to the length of the base (b) of Clara cells analyzed morphometrically decreased dependent on trichloroethylene dose. Electron microscopy showed that the bronchiolar luminal surface was covered by flattened and dilated endoplasmic reticulum and that destruction of mitochondrial crista and disappearance of secretory granules were dependent on increase in trichloroethylene concentration. Time-course studies conducted with exposure of 8,000 ppm of trichloroethylene to ethanol-treated rats demonstrated that maximal Clara cell damage occurred between 8 and 22 h after exposure, and that the cells appeared to be virtually identical to these of the control by 4 wk after exposure. Ethanol ingestion slightly increased Clara cell damage induced by trichloroethylene.