The chapter is adapted from the recent proposal by the Interagency Pharmacokinetics Group (IPG) advocating the adoption by the participating Federal regulatory agencies of a unified method of scaling daily administered amount of a carcinogenic agent in proportion to the 3/4 power of body weight. The dose-scaling approach was proposed as a default assumption, to be used when case-specific data are inadequate, to achieve presumed equal lifetime cancer risks in different mammalian species following lifetime exposure. In this chapter, the empirical investigations of how carcinogen potencies vary among species are reviewed and the implications for choice of a general risk assessment methodology for carcinogen dose scaling are discussed. The empirical data on comparative carcinogenic potencies in different species support the general practice of scaling rodent potencies to humans, and show that, on average, current methods perform rather well. The data are not of sufficient resolution, however, to distinguish between surface area and body weight dose scaling.