In order to determine the effects of continuous exposure to 17 ± 2 ppm NO2 in air on cell renewal in bronchiolar and alveolar cells, young rats were exposed continuously for as long as seven days, removed at intervals, and killed. One hour before the killings, dividing cells were labeled with tritiated thymidine. In terminal bronchioles, labeling index increased fourfold, the mathe first 24 hours. The index decreased slowly and by seven days had returned to the original level. In the alveolar walls, the labeling index increased fourfold, the majority being labeled type II cells. Here, too, the labeling indexes had returned to control levels by the seventh day. Cell renewal, thus, is initially stimulated by NO2 and then returns to control levels.