Advances made in recent years in laboratory methods for the study of kidney function have extended our interests beyond the observation of structural changes. The finding of albumin, casts, blood or pus in the urine, is in the great majority of cases definite evidence of structural changes in the kidney. Newer laboratory methods have, however, taught us that anatomic and functional integrity of the kidneys are not synonymous terms. Serious functional changes may be found clinically, and at necropsy no commensurate degree of tissue destruction may be noted. Extensive tissue destruction may be found at necropsy, and with our present methods of determining functional activity, no marked impairment may be found clinically. Much is still unknown regarding the physiology of the kidney. For this reason any new method devised as a means of determining kidney function is welcomed by the clinician. In February, 1920, MacLean and de Wesselow 1 reported a