The chromatographic separation of cortisone, cortisol, 9a-fluorocortisol (free alcohol and acetate), prednisone and prednisolone is described, and Rcortisone-values are given. The comparative rates of metabolism (disappearance of the conjugated unsaturated system of ring A, of the a-ketol side chain and of the steroid identified by paperchromatography) of the above mentioned steroids with rat liver slices have been measured by studying the reaction rate at varying times of incubation. The following relation has been found: Destruction of cortisone=100, cortisol=93, fluorocortisol=69, prednisone=43 and prednisolone=27. These findings are discussed in connection with the differences among these steroids in biological and clinical activity and their mode of therapeutic application. The conversion has been observed of cortisone to cortisol, of prednisone to prednisolone, but not the reversal of this reaction. Other not yet identified metabolites are shown in chromatograms. The specificity of hydrogenases engaged with the hydrogenation of the 3-keto- A -structure, and C11 and C20-keto groups are discussed.