A quick method for distinguishing among the possible causes of oliguria would be of great value in deciding on treatment. The method here proposed depends on simultaneous determinations of the concentration of urea nitrogen in serum and in urine. In 18 surgical patients the ratio of urea nitrogen in the urine to that in the serum ranged from 14 to 113. In 12 patients with chronic renal insufficiency it ranged from 1 to 13. Data are also presented on 43 patients in whom azotemia was either transient or fatal in connection with illness or surgical operations. Values below 10 were an ominous prognostic sign. Since the test is simple enough for routine use in a chemical laboratory, it is useful as a therapeutic guide. When the index is high, a generous intake of water is permissible.