Rats of similar age and size were exposed continuously to nitrogen dioxide (NO2) at a concentration of about 15 ppm in air for 1, 2, 3, and 4 wk and 2, 3, 4, 5, 7, 13, 15, and 17 months. Large increases in lung volume and lesions of the small airways and their epithelium and that of adjacent alveoli developed. The lesions in the terminal bronchioles involved hypersecretion and asggregation of cellular debris and free cells in their lumena. A calibrated grid fitted to an eyepiece was used to measure the diameters of all patent bronchioles at the proximal point of the first alveolar "break" in the respiratory bronchiole. The terminal bronchioles and their short respiratory bronchioles inthe NO2-exposed animals developed stenosis, which increased with time. The maximum change, occurring after 17 months of exposure, was a 45.6% reduction in bronchiolar diameter. Over-all constriction was underestimated because nonpatent sections of bronchioles were excluded The ratio of lung volume to bronchiolar diameter in exposed animals also increased with time as a result of the rising volume and the falling bronchiolar diameter. Probable are discussed.