Activity of the cilia of the upper airway epithelium is one of the most important biological self-defense actions against the inhaled gases, fumes and particles. However, there are only a few systematical reports on the influence of air pollutants on the ciliary activity. The authors developed a new system to measure the ciliary activity as the beat rate in vitro, applying the phase-contrast microscope, photoelectric cell and oscillograph, and exposure chamber for the continuous ventilation of a gas of a certain concentration. For SO2, NO2, NO, HCHO and O3, as air pollutant gases, the influence of these gases on the ciliary activity of the excised specimen of the trachea of the guinea pig were experimentally studied. To compare the influence of each gas, the idea of Half Reduction Time of ciliary activity by a gas of certain concentration was introduced and the time of each gas for 50 ppm concentration was: SO2 , 4 min; HCHO, 5 min; NO2 , 10 min; NO, 14 min; and O3, 20 min. It can be assumed that the influence of the easily water-soluble gases such as SO2 and HCHO, on the ciliary activity is severer compared with the less water-soluble ones, NO2 and O3 , as the latter gases pass over the mucous surface of the upper airway, without dissolving to the mucous layer.