The state of the water-electrolyte metabolism was studied by the radio-isotope dilution technique in 115 patients with different forms of ischaemic heart disease and in 25 normal persons. It was established that all forms of ischaemic heart disease are accompanied by a significant potassium deficit imperceptible for blood and urine examinations of their electrolyte composition. The potassium deficit is most clearly expressed in patients with cardiosclerosis, especially in congestive heart failure. In a considerable part of the patients with acute myocardial infarction, including those without clinical signs of congestive heart failure, the rate of sodium and water metabolism is decreased, and the total body content of metabolizing sodium is increased. Sodium retention in the body is especially severe in patients with cardiosclerosis complicated by congestive circulatory insufficiency. The administration of diuretics in adequate doses was shown to result in normalization of the indices of sodium and water metabolism.