I. The renal clearance of mannitol is frequently less than the simultaneous clearance of inulin in man. 2. The intravenous injection of manitol depresses the inulin clearance in man. 3. The clearance of inulin is equal to that of creatinine in the dog, but the mannitol clearance is usually less than that of creatinine. 4. mannitol undergoes metabolic alteration in man, and therefore, cannot be utilized in the infusion pump technique for measuring the rate of glomerular filtration. 5. Inulin appears to be the most generally useful substance for the measurement of glomerular filtration in man.