It is not the intention here to add another somewhat complicated procedure to the already rather formidable list of tests of renal function. However, on the basis of the work of Wakefield, Power and Keith,1 confirmed by Hayman and Johnston,2 showing that the concentration of inorganic sulphate in the blood is often elevated before the urea, the sulphate clearance should show a change before the urea clearance and should give still earlier evidence of impaired renal function. Here, of course, is met the question of renal physiology. How is inorganic sulphate eliminated by the kidney? In what way does its excretion differ from that of urea or creatinine? Certain valuable and useful laws governing the excretion of urea, leading to the urea clearance test, have been established without knowledge of the exact histologic mechanism by which it takes place, and the same may be true for sulphate.