The efficiency with which diodrast (3, 5-diiodo-4-pyridone-1-acetic acid) and hippuran (oiodohippuric acid) are excreted by the renal tubules (1, 2) raises the question to what extent the organically-bound iodine in these molecules is responsible for the phenomenon of tubular excretion. Since suitable quantitative methods for the determination of unsubstituted compounds wvere not available, substituted derivatives of hippuric acid which could be determined by appropriate coupling reactions were prepared 2 and studied under conditions permitting the exact comparison of renal clearances with those of diodrast atnd hippuran.Methods of comparing clearances. When two substances, both of which are excreted by the tubules, are presented to the tubular excretory mechanism simultaneously, one substance may depress the tubular excretion of the other (2, 3).3