Extending previous studies, we hypothesized that glycerol modulates pulmonary tumorigenesis in mice through metabolic activation in bronchiolar cells. To test this, we examined the effects of glycerol on pulmonary cell kinetics and tumorigenesis induced by urethan. Male ddY mice were given 1 mg/g urethan and/or 5% glycerol solution for a fixed period. [3H]Thymidine autoradiography revealed that glycerol administration significantly suppressed the urethan-induced facilitation of cell kinetics of the bronchiolar cells (p less than 0.05), but not that of the alveolar wall cells. Furthermore, glycerol did not affect urethan-induced pulmonary tumorigenesis. These findings suggest that glycerol modifies the metabolism of carcinogens in the bronchiolar epithelium.