Nitrogen dioxide, sulfur dixide and carbon oxide are major air pollutants which closing up as prevalent gases in our artificial environments recently. As it has been well known, these gases are classified as irritant gases and its effects by inhalation have been reported by many authors on the intoxication. Recently several experimental studies to evaluate the pathological changes of respiratory systems by inhalation of these gases were performed in our department and author summarised as follows ; 1) Generally, severe pathological changes were observed in the higher concentration group even if product of time and concentration were equal. 2) Lung edema was observed in the 2,000 ppm SO₂ group. 3) In the high concentration SO₂ group, the remarkable pathological changes were observed in the upper respiratory tract and the most remarkable changes were found in the carina and main bronchi. In the 100 ppm Exposure group, pathological changes were severe in the lower respilatory tract. 4) In the high concentration and short duration SO₂ exposure group, suppurative or mucoid exudate was observed contrary to this finding, in the low concentration and rather long duration exposure group, hemorrhagic change was observed. 5) The pathological findings of 500 ppm NO₂ group were simillar to the 1,000 ppm group. 6) In the 100 ppm NO₂ group the severity of the pathological changes were almost same as those of 1,000 ppm NO₂ group. This finding might be due to the longer inhalation time imposed to 100 ppm group. 7) In the interpretation of dose-response relationship, the concentration and time should be taken into consideration with a reasonable extrapolation.