Conditions are described for the measurement of cortisol, cortisone, prednisolone and prednisone by gas chromatography. A procedure is outlined for measuring them and some of their unconjugated metabolites in urine. The procedure involves the separation of the urine extract into four paper chromatographic fractions, oxidation of each fraction with sodium bismuthate and gas chromatography of the oxidation products. This procedure is considered to be preferable in several respects to the methods currently used for the same purpose.