Abstract Injury and cellular proliferation in the lung were examined following administration of 1,1-dichloroethylene (1,1-DCE) or vinylidene chloride. C57BL/6 male mice were treated orally with 200 mg/kg of 1,1-DCE prior to a single pulse of tritiated thymidine ([ 3H]TdR). Necrosis and exfoliation of Clara cells of bronchiolar epithelium were evident by 1 day after chemical administration, and increased in severity by 2 days. A regenerative response was observed at 3 days after 1,1-DCE administration, and by 7 days the epithelium was substantially restored. At 30 days after 1,1,-DCE re-epithelization was achieved and areas devoid of epithelium were not observed. Changes in cellular proliferation were calculated from measurements of [ 3H]TdR incorporation into total pulmonary DNA. Activity of [ 3H]TdR was significantly inhibited at 1 day after chemical administration and there-after increased: a peak of synthesis occurred between 3 and 5 days. At 7 days after 1,1-DCE administration, incorporation of [ 3H]TdR decreased to levels that were not significantly different from those of control animals. Autoradiographic examination of 0.5 μm thick plastic-embedded lung sections showed that [ 3H]TdR was incorporated into the DNA of bronchiolar epithelial cells, macrophages, interstitial, endothelial and Type II alveolar cells. However, the majority of the label was taken up by the nonciliated bronchiolar epithelial cells. The increased [ 3H]TdR incorporation into whole lung correlated with repopulation of bronchioles which was observed following injury. The results demonstrated that 1,1-DCE-induced damage to Clara cells of the bronchiolar epithelium was severe and rapid; re-epithelization was achieved in a relatively short time whereas differentiation was a prolonged process.