The radioisotope renogram, which has now been used for a decade as a means of testing kidney function, is enjoying world-wide acceptance. Great demands have been made on it as a diagnostic tool, even though its design does not provide for visualization of the kidneys and renal arteries. Essentially, it was developed as a means for estimating and comparing blood flow and function of individual kidneys, and the ability of the upper urinary tract to evacuate urine. Repeated attempts have been made to introduce quantitative interpretations of the test results; unfortunately these only complicate the use of the test. Numerous formulas have been devised to this end, most of which depend on the time intervals to the points of maximal amplitude and descent halfway to the baseline of the tracing from its beginning. The application of such formulas to the interpretation of results assumes, as in the case of computers,