The present study was performed to relate Clara cell proliferation and Clara cell morphology to duration and intensity of nitrogen dioxide (NO2) exposure as a model of lung injury in rats. Male Sprague-Dawley rats were exposed to 5, 10 and 20 ppm NO2 for three (short term exposure) and 25 days (long term exposure). Epithelial proliferation was immunohistochemically assessed by BrdU-incorporation into the bronchial and bronchiolar epithelium. Clara cell specific 10 kD protein was detected using an antibody versus recombinant protein. Following short term exposure Clara cells showed a significant alteration of their morphology: the cells lost the apical intraluminal projections, and the damaged epithelium was covered by a layer of CC10 reactive material. After long term exposure the Clara cell morphology was indistinguishable from that of unexposed controls. In all short term exposure groups cellular proliferation was higher in the bronchiolar epithelium than in the bronchial epithelium. Whereas short term exposure to 5 ppm NO2 sufficed to induce a maximum epithelial proliferation of the bronchiolar epithelium, long term exposure to 5 ppm NO2 yielded no significant increase in the epithelial proliferation, a finding reflecting the adaptation of the airway epithelium to oxidative stress. Long term exposure to 10 and 20 ppm NO2 resulted in a dosilinear increase of cell proliferation in the bronchial and bronchiolar epithelium. Double labelling of CC10 and BrdU showed that BrdU uptake and hence proliferation were restricted to Clara cells. We conclude that changes of the Clara cell morphology are closely related to the extent of inhalative stress and functional status of the Clara cell. The results are in keeping with the assumption that the Clara cell exerts progenitor cell functions in the airway epithelium.