1. Total body water, extracellular fluid, total exchangeable potassium and total exchangeable sodium were measured by the multiple isotope-dilution technique in eight patients with congestive cardiac failure. Measurements were made on each patient while in failure and then again after treatment and recovery. 2. Compared with five control subjects in whom the same measurements were made, patients in failure had an excess of total body water, extracellular fluid and total exchangeable sodium, and had less total exchangeable potassium. 3. There was a decrease in total exchangeable sodium and an increase in total exchangeable potassium on recovery from congestive cardiac failure 4. After treatment of cardiac failure, intracellular potassium increased and intracellular sodium was decreased. 5. The low total exchangeable potassium in cardiac failure is not simply due to loss of lean body mass.