Rats were exposed continuously for a natural lifetime, except for periods of respite, to approximately 15 ppm of nitrogen dioxide, a concentration that did not induce edema, hemorrhage or inflammation. The aged, sacrificed animals had voluminous, dry lungs with a large functional residual capacity. The lung volume was greater than that in control animals at maximal standard pressure. Terminal bronchioles and alveolar ducts exhibited loss of cilia, epithelial hypertrophy, and narrowing. Alveoli were distended with attenuation of walls in the periphery and hypertrophy of epithelium in central alveoli. Exposed animals had approximately one-third the number of alveoli of control animals, and the ventilatory surface area was reduced accordingly. In the advanced state of disease, arterial oxygen tension was reduced and secondary polycythemia was apparent. The emphysema-like disease resembled that in man.