Five balance studies in four patients with congestive heart failure have shown that the positive electrolyte balance and normal serum electrolytes are significantly influenced by various diuretics. Given alone, hydrochlorothiazide produces negative electrolyte balances and appropriate reductions in serum electrolyte levels. When hydrochlorothiazide and triamterene are given concomitently, the potassium and chloride losses are greatly minimized, whereas the sodium loss is accentuated. In the dosage used in this study, furosemide is a potent diuretic producing minimum potassium wasting. Triamterene not only enhances the natriuretic property of furosemide but also accentuates potassium conservation. We stress that cardiac arrhythmias are frequently associated with the use of hydrochlorothiazide but rarely with the use of furosemide. Triamterene usually restores lost potassium and, thereby, effects disappearance of the arrhythmias. Potassium metabolism plays a dominant role in the pathogenesis of these arrhythmias.