A procedure permitting simultaneous calculation of the isotopic content and the mass of steroid metabolites produced upon incubation of a radioactive precursor with rat adrenal tissue was applied to the steroids produced from progesterone or 11-deoxycorticosterone in incubations of whole and of quartered adrenals.The sp. activities of all the steroids studied were much smaller in the tissue than in the media for whole as well as for quartered adrenals, and progesterone-4-14C was little diluted by endogenous progesterone.Thus, several metabolic pools of steroids may exist which do not readily mix, and cell membrane permeability to different steroids may differ.Aldosterone and 18-hydroxy-corticosterone production by whole adrenals was almost twice that by adrenal quarters, but the reverse held true for corticosterone and 18-hydroxy-11-deoxycorticosterone.Thus, the latter 2 steroids may be utilized by glomerulosa cells as precursors of 18-hydroxycorticosterone and aldosterone, and to a greater extent by whole than by quartered adrenals.