The aspect of Mg++ metabolism was studied in 83 patients with congestive heart failure (CHF). The result indicated that CHF per se and treatment with digoxin were not the causes of Mg++ depletion. Pronounced diuresis induced by diuretics increased markedly renal excretion of Mg++ and, in consequence, lowered the plasma Mg++ in 55% of the patients. The hypomagnesemia was not associated with decreased RBC Mg++, and was usually transient and asymptomatic. When in addition to diuretics there were other factors known to promote extra Mg++ loss, the hypomagnesemia became more severe, the RBC Mg++ level might be decreased in association with it. A part of these patients might develop symptomatic Mg++ deficiency, the chief clinical manifestations of which included changes in neuromuscular excitability, mental disturbances and cardiac arrhythmia, often accompanied by hypopotassemia, hypocalcemia and metabolic alkalosis. The important role played by the Mg++ depletion in causing relevant symptoms and other electrolyte disturbances might be identified by careful analysis of the whole situation of the patient and the therapeutic test with Mg++. The lowering of the plasma and RBC Mg++ levels, the decrease in the 24 h urinary Mg++ excretion could be used as routine laboratory diagnostic criteria of Mg++ deficiency, and the application of the Mg++ loading test as an important diagnostic procedure and at the same time a therapeutic test should be used more frequently in the future.