The effectiveness of magnesium therapy in intractable ventricular tachycardia and ventricular fibrillation was documented in patients not only with hypomagnesemia, but also in patients with normomagnesemia. It was also effective in ventricular tachycardia characterized by 'torsades de pointes' and in massive digoxin intoxication. Prospectively, parenteral magnesium therapy was also effective in controlling the ventricular rate in multifocal atrial tachycardia (8 patients) by reducing the number and the rate of ectopic atrial foci. Parenteral magnesium sulfate administration, 10-15 ml of 20% MgSO4 in 1 min and 500 ml of 2% MgSO4 in 5 h in patients without renal failure was found to be safe and effective. Magnesium sulfate prevented hyperpotassemia in massive digoxin intoxication and tended to produce hypopotassemia in other patients necessitating concomitant use of potassium chloride.