Nitrogen dioxide is one of the most prevalent noxious gas in our artificial environments. Toxicologically, nitrogen dioxide is classified as an irritant gas and its effects by inhalation have been ascertained by many case reports on the intoxication incidence in the industrial exposure. A strong irritating effect on the epithelium of airway tract and the formation of methemoglobin have been known as the main toxic action. Clinically, pulmonary edema and cogestion are believed to be main causes of death. Author designed this study to elucidate the pathological findings at the different sites and levels of the repiratory tracts of the rats exposed to the nitrogen dioxide of 1,000 ppm, 500 ppm and 100 ppm in concentrations. The findings observed are as follows : 1) Average tolerance of rat to 1,000 ppm nitrogen dioxide was 5.3 hrs. and in case of 500 ppm, it was 6.8 hrs. 2) In the 1,000 ppm group, loss of cilia, squamous cell metaplasia, basal cell hyperplasia, goblet cell hyperplasia, edema and congestion, inflammatory infiltration, epithelial ulceration and mucous gland hypertrophy were found in the larynx, trachea, carina, main bornchus and lobe bronchi. Lymphoid hyperplasia was found in the lobe bronchi. In the lungs diffuse and marked edema were noticed and moderate congestion with bronchiolitis and acute bronchopneumonia were observed. 3) In the 500 ppm group, the severity of the pathological changes were much less than those of the 1,000 ppm group, but showed the similar findings. 4) In the 100 ppm group the severity and the characteristics of the pathological changes were almost same as those of 1,000 ppm group. This finding might by due to the longer(almost 12 times) inhalation time imposed to 100 ppm group. Thus, in the interpretation of dose-response relationship, the concentration and time should be taken into consideration with a reasonable extrapolation.