Histopathological and biochemical examinations of the nasal cavity and lungs of rats inhaling 145.6 ppm (high-dose) or 15.0 ppm (low-dose) formaldehyde vapor for 6 h revealed dose-related damage. The contents of non-protein SH groups (NPSH) in the nasal mucosa and lung, and lipid peroxide (LPO) in the nasal mucosa were decreased, whereas LPO was increased in the lung. The contents of triglyceride in the lung tissue and lavage, and free cholesterol in the lung were decreased, providing evidence of a suppression of surfactant production. Exposure-related changes in the nasal turbinates, trachea, and lung included hyperkeratosis of the squamous epithelium, increased secretion, and desquamation of ciliated and mucosal cells. Under the present experimental conditions, 145.6 ppm formaldehyde vapor was found to strongly affect the exposed tissues. Changes in lipid peroxide were dependent on the level of exposure.