The ability of the renal tubules to concentrate urine is the basis of several clinical tests of renal function in use today. Fishberg1 expressed the belief that the specific gravity tests are the most useful tests of renal function available for the general practitioner and general hospital use. Sodeman and Engelhardt2 recently advocated use of solution of posterior pituitary for a renal function test. This test utilizes the antidiuretic principle of the posterior lobe of the pituitary gland and enables one to perform a renal concentration test without prolonged restriction of the intake of fluids. It has the further advantage of giving reliable results in the presence of ascites or cardiac edema, where other concentration tests cannot be used because of the impossibility of depriving the kidneys of the excess fluid already available in the body. The latter point is important in deciding whether the function of the