Diuretic drugs have been the mainstay of treatment for heart failure. Specific elucidation of the effects of diuretics on electrolytes, however, has been difficult, since the heart failure state itself may alter the electrolyte balance. Nevertheless, it is noteworthy that the natriuretic effect of loop diuretics is greater in edematous patients than in healthy volunteers; yet, the initial kaliuresis is minimal--perhaps because aldosterone levels are low. With continued treatment (or after the edema has been cleared), however, the natriuretic action of loop diuretics is less than that seen in controls, but loss of potassium occurs. The addition of a thiazide to a loop diuretic enhances its natriuretic effect. Both loop and thiazide diuretics can induce depletion of magnesium. Potassium-sparing diuretic drugs augment the natriuresis induced by loop or thiazide diuretics but limit or prevent the loss of potassium or magnesium in the urine, at least in the short term. However, potassium-sparing diuretics can exacerbate the development of hyponatremia.