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.

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Two groups of depressed subjects, one with a history of recurrent depression, the other with a history of persistent apathy, were given lithium carbonate 1,200 mg q.i.d. and supplementary potassium 1,200 mg t.d.s. for 1 week. Measurements were made before and after the lithium treatment of total body water (tritium space), extracellular fluid (sulphate space), total exchangeable sodium (Na_e) and total exchangeable potassium (K_e) using sodium-24 and potassium-42 multiple isotope dilution techniques. Prior to treatment when compared with a group of normal subjects, both depressed groups showed changes in body fluid volumes and electrolyte levels. Total body water, intracellular fluid and intracellular potassium were lowered, while intracellular sodium was raised. After treatment with lithium the values in the apathetic group showed little change but the group with recurrent depression showed a significant increase in intracellular fluid (p < 0.025), K_e (p < 0.001), intracellular potassium (p < 0.025) and a significant decrease in Na_e (p < 0.05). There was a marked increase in mood in the group with recurrent depression but not in the apathetic group following lithium treatment. These findings suggest that recurrent depression, both in clinical improvement, mood and also correction of water and electrolyte disturbances arise, but not in patients with long-standing apathy.