Cytochrome P-450-dependent monooxygenase systems, which metabolize endogenous as well as foriegn compounds, are found in hepatic and several extrahepatic tissues of mammals, including humans. A form of cytochrome P-450 is localized in the nonciliated bronchiolar epithelial cells (Clara cells) of the small airways of rabbit lung. The apparent high concentration of the cytochrome in this pulmonary cell type compared to liver may be an important determinant in the susceptibility of the lung to a number of toxic chemicals that undergo metabolic activation.