There are many tests of kidney function. Some of them are fundamentally unsound because the kidneys are not working to their fullest capacity at the time of the test. The most commonly used test belonging in this category is the estimation of the blood nonprotein nitrogen. Addis1has shown that the activity of 50 per cent of the kidneys is sufficient to remove the nitrogenous wastes at the normal rate. It follows that the nonprotein nitrogen of the blood will not be elevated until the disease has destroyed at least a half of the kidneys. The test is, accordingly, inadequate at the time when it is most needed. In order to obtain the information desired earlier in the disease, Addis2proposed to put the kidneys under a maximal strain by giving large doses of urea and by forcing fluids. By comparing the ratio of urea in a one