Ultrastructural lesions were induced by formaldehyde (HCHO) gas in the rat nasal respiratory epithelium. Male F-344 rats, 7-9 weeks old, were exposed to 0.5 or 2 ppm (6 hr/day) for 1 or 4 days and to 6 ppm (6 hr/day) of HCHO for 1 day and sacrificed immediately or 18 hr after 1, 2, or 4 days of exposure. Other groups were exposed to 15 ppm (6 hr/day) of HCHO for 1 and 2 days. Ultrastructural changes to 0.5 or 2 ppm were limited to altered cilia with wing-like projections occasionally seen on the tips of the ciliary shafts. Autophagic vacuoles were present in some of the basal cells while neutrophils were seen in the basal and suprabasal layers after 1 day exposure to 6 ppm of formaldehyde. Hypertrophy of goblet and ciliated cells were noted in animals exposed to 6 ppm of formaldehyde and sacrificed 18 hours after 1, 2, or 4 days of exposure. Some nonciliated cells formed apical blebs containing an abundance of SER. Ciliated-mucous cells were observed after 2 and 4 days of exposure to 6 ppm of formaldehyde. Nonkeratinized squamous cells containing microfilaments were seen as early as 4 days after exposure to 6 ppm and at 1 and 2 days after exposure to 15 ppm. Loss of microvilli in ciliated cells occurred in all exposure levels. At 15 ppm for 1 and 2 days, nucleolar segregation was observed in basal and cuboidal cells and internalized cilia were noted. These results demonstrate that short-term exposure to 6 or 15 ppm of HCHO caused respiratory epithelial injury which was not cell specific, but was dose related in severity.