The purpose of the present study was to identify the proliferative cell types in the nonciliated cell population of the upper airways and determine the capacity of each to act as progenitor cells. Sprague-Dawley rats (30 days old) were exposed to 20 ppm NO2 for 24 hours to stimulate cell division, given injected tritiated thymidine (3H-TdR), sacrificed 1 hour and 1, 3, 5, and 7 days later, and prepared for light-and electron-microscopic autoradiography. One hour after injection of 3H-TdR, the mean labeling index (LI) was 1.6% in control animals and 5.2% in exposed animals. Mean grain counts per cell decreased from 15.6 at 1 hour after 3H-TdR to 6.9 on the third day, indicating that the labeled cell population had divided. Labeled cells in the control and exposed cell populations were identified with electron microscopy. At 1 hour after injection of 3H-TdR, basal cells and nonciliated columnar cells were labeled. However, only nonciliated columnar cells were stimulated to divide by NO2. The labeled nonciliated columnar cell population was made up of serous," intermediate" and goblet cells. Each of these cell types was stimulated to divide to the same degree. After cell division (1-7 days) labeled cells of all types were observed with labeled ciliated cells appearing on the third day. It was concluded that the basal cell is not a primary progenitor cell. The primary progenitor cell for epithelium in the upper airway is the total columnar secretory cell population (serous," intermediate," and goblet cells).