The authors examined the dynamic properties of histamine inside the lungs and other organs through repeated exposures to NO2 of relatively low concentrations over a long period, and also attempted sthenometric measurements by means of the natation method. With the lung histamine content, some increase was observed in the 5 ppm group on the 20th day, while an increase was detected in the 1 ppm group on the 30th day. The histamine content of the blood was in ultra-micro quantities, but a gradually increasing tendency was indicated in the 1 ppm group, while no change was observed in the 5 ppm group. In the examination of the mice, changes accompanying the progress in the exposure to NO2 were not observed in terms of the lung water content, but in the sthenometric tests certain reductions in the fatigue time and death time were observed.