The aim of any test of biologic function is to determine the existing function of an organ or tissue in relation to the function if the tissue or organ and organism as a whole were normal. A quantitative consideration is necessary for the proper evaluation of any function. In the case of the kidney, an attempt to measure the amount of functioning renal tissue must be the aim of any test of function. The test with phenolsulphonphthalein (phenol red) is probably the most widely used, at least in this country, of the numerous tests of renal function which have been proposed, and it is the purpose of the present study to inquire to what degree it meets the criteria. When the test was first described by Rowntree and Geraghty,1 it was found experimentally that the removal of half the normal renal tissue was without influence on the results of