The elimination through the kidney of a fixed proportion of ingested or parenterally administered pentose sugars, combined with the extremely low threshold of the kidney membrane for these sugars, would seem to indicate their use as a delicate measure of renal function. Up to this time, such a use has not been feasible owing to the extremely high cost, but the recent development of a method for the cheap production of xylose has rendered that substance available for this purpose. One thus is enabled to judge the functional power of the kidney by a simple quantitative determination of the reducing-power of the urine two hours after ingestion. Somogyi's1 simple method for the determination of a nonfermentable reducing substance in the blood in the presence of dextrose by the substitution of a yeast suspension for the water used in the Folin-Wu tungstic acid precipitation in the determination of the blood