This study using an airway model was carried out to compare the dynamic behavior of SO2, NO2 , and SO2-NaCl aerosol mixture in the respiratory system after the inhalation. The model airway was constructed as follows: The inner wall of a glass tube was lined with moistened filter papers. Six such glass tubes were connected in series, and the last tube was connected with two bubblers in series. After SO2, NO2, or SO2-NaCI aerosol mixture was passed through the said glass tube-bubbler system for 15 minutes, the amount of SO, or NO, absorbed by each glass tube and the bubblers was measured. The following results were obtained: (1) Most of SO2 was absorbed by the first glass tube, and only a little by the distal glass tubes. Only a small amount of SO2 reached the bubblers. (2) Only a small quantity of NO2 was taken up by the glass tubes, and consequently, most of it reached the bubblers. (3) As for the SO2 and NaCl submicron-sized aerosol mixture, the pattern of SO2 absorption by the glass tubes was almost similar to that observed for SO2 alone, but a considerable amount of SO2 was detected in the bubblers. These results strongly support the generally accepted hypotheses that (1) a respiratory effect of SO, develops immediately after being absorbed by the upper airway, (2) NO2 can reach the lungs producing delayed effect on it without being absorbed at the upper airway, and (3) the submicron-sized aerosol might have the capacity to adsorb SO, and carry it into the deeper airway.