My experience with the use of phenolsulphonephthalein commenced very shortly after Rowntree and Geraghty1began their experiments with it. I base my belief in its efficacy on a large number of cases seen in Dr. H. H. Young's clinic at the Johns Hopkins Hospital and in my private work. I shall also consider here some of the other functional tests of the kidneys, and my purpose in taking them up is more to show their defects than to set forth their manner of use or for the observation of their individual values. I do this that the value of phenolsulphonephthalein may be more readily seen. THE PHYSIOLOGIC FUNCTION OF THE KIDNEYS The physiologic function of the kidneys is to eliminate from the body certain substances brought to them by the blood. This is generally supposed to be done by secretion, as defined by the Bowman-Heidenhain theory, rather than by