Using Na 22 an unexpected magnitude of Na retention was found in a patient with congestive heart failure. The best criterion for evaluation of this is by means of comparing "Na Clearances". The normal control was found to excrete 50 times as much sodium as the patient in failure when compared in terms of clearances. The Na intake caused these differences to vary directly, i.e., with greater intake there was a greater difference. This valuable tool for investigation is relatively simple to employ.