The intrinsic clearance of unbound drug (CLuint) for antipyrine in 15 mammalian species was characterized by an equation of the form, CLuint = theta 1 (body weight) theta 2 (brain weight) theta 3, where thetas are constants. Maximum lifespan potential in mammals can also be characterized by an equation of this form. It is suggested that the set of genetic mechanisms regulating longevity and other constitutional characteristics in species is also linked to rates of drug metabolism. The ability to scale interspecies data in pharmacokinetics is taken as an expression of this design principle.