The role of calcium and potassium in myocardial irritability and contractility has been well studied. More recently the interrelationship between these ions and digitalis has been explored. The concept of synergism between, or additive effect of, calcium and digitalis has been debated for many years.1-4The use of potassium salts orally and parenterally in the treatment of digitalis intoxication is well established.5,6It is known, in addition, that low serum calcium levels potentiate the cardiac toxicity of hyperkalemia.7 Digitalis is known to cause egress of potassium from the myocardial cell.8It is this relative intracellular potassium depletion which may, in part, be responsible for the commonly observed electrocardiographic effects of digitalis.9Calcium ions at the cell membrane exert a stabilizing influence, making more difficult the active reentry of potassium.10This results in the same net effect. It seemed reasonable to expect that lowering the concentration of calcium ions at the