It has been well known that Carbon Monoxide and Sulfur dioxide are ubiquitous noxious gases and are consisting of the major gaseous air pollutants. Many research works and reports on the synergic effects of those two gases suggest this subject appear to be important in the field of the environmental pollution. Author designed this study to elucidate the inhalation effects of simple CO, SO₂ gases and mixed CO-SO₂ gas on the respiratory tract of the rats in terms of the pathological changes by the location of the airway tract. Four experimental groups, those of 5 ppm SO₂, 10 ppm SO₂, 1,000 ppm CO and 1,000 ppm Co-5 ppm SO₂ groups were exposed to each concentration for 3 weeks with daily exposure of 30 minutes. Various pathological findings were observed at 7 different sites of airway tract. The results obtained are as follows : 1) In the 5 ppm SO₂ exposure group, patholgical changes namely loss of cilia, inflammatory infiltrate, edema and congestion were remarkable in the carina, main bronchi 10 ppm SO₂ exposure group showed similar pathologic changes as 5 ppm SO₂ group with somewhat enhanced severity but the degree of changes did not followed concentration gradient. 2) In the 1,000 ppm CO exposure group, no remarkable pathological changes were found except mild degree of edema and congestion all along the airway tract. 3) In the CO-SO₂ mixed gas group, more remarkable pathological changes than those in SO₂ gas group were found, especially in the carina, main bronchi, lobe bronchi and parenchyma of lungs. Among the pathological findings, sanguinous and mucous luminal contents, loss of cilia, inflammatory infiltrates, epithelial ulceration were severe and edema, congestion, interstitial fibrosis, intraalveolar hemorrhage and bronchiolitis were more marked intensive in degree of changes than those of 5 ppm SO₂, 10 ppm SO₂ and 1,000 ppm CO exposure groups especially in the lobule bronchi. 4) The synergic pathological effects of CO-SO₂ mixed gas on the respiratory tract was speculated as the cause of above mentioned results.