The electrocardiogram is an extremely sensitive method of detecting certain types of electrolyte imbalance. The form of the normal electrocardiogram depends upon the normal ionic constitution of the cells and, particularly, of the extracellular fluid bathing the cardiac cells. Any significant alteration in the electrolyte content of this fluid, or in the ratio between them, may directly or indirectly produce significant electrocardiographic changes. The electrolytes which may produce the most profound effects of the electrocardiogram when they deviate from their normal levels are calcium and potassium.* Changes have also been reported following alterations in magnesium levels.7 A rise or fall of sodium content per se does not affect the electrocardiogram, but alterations in this electrolyte may indirectly change the potassium effect by inducing acidbase imbalance and by altering the potassiumto-sodium ratio.8 It should be remembered that the electrocardiogram reflects changes as they exist in the heart. Although these