The original technic of Heard, Sobel and Venning (1946) was abandoned because of unspecific chromogenic urinary substances interfering with the colorimetric measurements; the benzene-water partition used in the method of Talbot et al. (1945) offered no advantage. The main steps in the modified method are the following Acidification of the urine to pH 1 by H2SO4, immediate extraction with chloroform, application of Girard''s separation, and colorimetric detn. of the reducing corticoids in the ketonic fraction by means of phosphomolybdic acid reagent, and measurement of the absorption at 700 m[mu] in a Coleman spectrophotometer, desoxycorticosterone being used as the standard of reference. Investigations on the different steps used in the analysis were carried out to establish the optimal conditions. The relation between the reducing ketonic corticoids and the biologically active glucocorticoids (glycogen-depletion test on adrenalectomized mice) was detd. in a large number of urines from ACTH-treated patients, and the agreement between the data is discussed. Both sets of results appeared to reflect the adrenal cortical function in a similar manner. The present modification of the method is suitable for routine clinical analyses.