Sulfur dioxide(SO₂) is well known as one of the most ubiquitous noxious gas and plays major role in the London type air pollution. In the physiologically classifications, sulfur dioxide belongs to the irritant gases and its irritating action induces various pathological changes along the respiratory tract and lung. Laryngospasm, lung edema and congestion has been known typical pathological changes. Since the world wide, interest on the air pollution has been getting higher, many experimental approach to the health effects of the major air pollutants attract many research workers in this field. Author designed this study to elucidate the relationships between concentration and exposure time in the pathological changes at the different sites of the respiratory tract of the rats. The product of time and concentration was fixed as 40,000 and 2,000 ppm, 1,000ppm and 100ppm of SO₂ with exposure time 20 min, 40 min, and 400 min. each were applied to 3 experimental groups, 7 sites of respiratory tract as nasopharynx, larynx, trachea, carina, main bronchi, lobe bronchi and lung were selected. And also 16 items of pathological findings were observed. As a result, following findings were observed. 1) Generally speaking, severe pathological changes were observed in the higher concentration groups and lung edema was observed only in the 2,000ppm group. 2) In the 2,000ppm and 1,000ppm exposure group, the remarkable pathological changes were observed in the upper respiratory tract up to the main bronchi, and the most remarkable changes found in the carina and main bronchi. Contrary to these findings, in the 100 ppm exposure group, pathological changes were severe in the lower respiratory tract. 3) It was found that the pathological changes in the respiratory tract exposed to sulfur dioxide showed three types of pattern by the anatomical locations and pathological changes. In the first type, the degree of pathological changes are directly proportional to the product of time and concentration. In the second type, the concentration factor is dominant and in the third type, exposure time plays more important role in the pathological changes.