Mice of several strains (A/J, SWR, MaMyJ, BALB/cByJ, 129J, and C57BL/6J) were treated with the carcinogens 3-methylcholanthrene, urethane, and 4-nitroquinoline 1-oxide and then given 1 or 5% glycerol in the drinking water for up to 4 months. Effects of glycerol on lung tumor multiplicity and incidence were evaluated. The effects of glycerol were variable, and in the majority of experiments glycerol failed to enhance tumor development in mouse lung. Analysis of cell kinetics did not show a proliferative response of alveolar or bronchiolar cells to glycerol. In rats, glycerol did not enhance the appearance of putative preneoplastic liver foci, and in C3H mice it did not increase the incidence of spontaneously occurring liver tumors. It is concluded that glycerol does not increase number or incidence of lung tumors in the mouse strains used, whether the animals are pretreated with a carcinogen or not. Glycerol does not affect liver tumor development.