A single ip dose of 1,1-dichloroethylene (DCE) to mice (125 mg/kg) caused a reduction within 24 hr in cytochrome P-450 and related monooxygenases in lung microsomes, with no corresponding changes in liver and kidney microsomes. Light microscopy revealed that at 24 hr, DCE caused a highly selective and complete loss of the bronchiolar nonciliated (Clara) cells at all levels of the tracheobronchial tree. Electron microscopy showed that at this time, the bronchiolar luminal surface was covered by flattened, elongated ciliated cells. Within 24 hr total microsomal cytochrome P-450 and NADPH cytochrome c reductase were maximally reduced to about 50% of control and cytochrome P-450-dependent enzyme activities decreased to about 60% of control. By contrast, coumarin 7-hydroxylase was reduced to approximately 10% of control within 4 days. Since pulmonary coumarin 7-hydroxylase has been shown to reside almost exclusively in the Clara cells, this finding is in agreement with the observed extensive necrosis of the Clara cells. The return of lung microsomal P-450-linked enzyme activities took between 3 and 6 weeks and was paralleled by a corresponding slow reappearance of the bronchiolar Clara cells.