The radiologic assessment of renal blood flow leaves much to be desired. The orthoiodohippurate renogram measures an integration of many variables including blood flow, tubular enzymatic function, tubular transport, and renal pelvic storage. The mercury renogram is even more complex. These technics are therefore useful only as an index of total renal function and not as a measure of renal blood flow. With the use of nonisotopic methods, the mean transit time of radiopaque contrast material affords an index of renal blood flow but is also limited by the fact that in addition to blood flow, renal function plays an important part in the changing kidney density. In 1963 Thorburn et al. (1) applied the Kety-Schmidt washout technic of inert radioactive gases to the assessment of renal blood flow, affording the opportunity to quantitate the intrarenal distribution and renal blood flow in cubic centimeters per 100 g perfused tissue per minute. The radioactive gas washout technic requires intra-arterial injection ...