A simple apparatus for the generation of formaldehyde (HCHO) from its standard solution and an animal chamber for the inhalation of HCHO were developed. A linear correlation between the HCHO concentrations of animal chamber and of standard solution was found. By use of these apparatus, the toxic potential of inhaled HCHO in bronchial tissue was investigated histoparhologically in mice. When animals were exposed to 4 or 8 ppm HCHO for 1 h, only the ciliated cells of bronchial mucosa were degenerated concentration-dependently while marked degeneration in both the ciliated and goblet cells was produced after 1 h exposures once a day for 4 days. In the isolated guinea pig trachea experiments, HCHO induced the contraction of tracheal smooth muscle concentration-dependently at 0.03-0.3% but the irreversible relaxation at 1%.