Metabolism of corticosterone to more polar steroids is markedly less dependent upon exogenous NADPH in liver homogenates of C57BL/10J mice than in those of DBA/2J mice (Lindberg, Shire, Doering, Kessler & Clayton, 1972). Similarly, studies of the metabolism of mevalonic acid to sterols in vitro (Gaskin & Clayton, 1972) indicated a greater availability of NADPH in liver homogenates from C57BL/10J mice than in those from DBA/2J mice. Physiologically, a difference in NADPH availability could have far-reaching consequences, since this cofactor participates in a wide range of synthetic reactions. Biochemically, the enzymic basis of the difference remains to be established. To broaden the base of future comparative and genetic work on these problems, we describe here the extension of this study to a total of six inbred strains of mice. The availability of NADPH was assessed by measuring the reductive metabolism of corticosterone in a modification of the system described by