The efficacy of magnesium sulfate (MgSO4) infusion in the treatment of ventricular arrhythmias was evaluated in 10 normomagnesemic patients: seven men and three women, aged 56-78 years (mean +/- SD, 63.8 +/- 9.3). All of the patients had ischemic dilated cardiomyopathy (IDC) and severe ventricular arrhythmias: multiform ventricular premature contractions (VPCs), couplets, runs of ventricular tachycardia (VT), and R-on-T phenomenon. Four had evidence of old myocardial infarction (MI), four had chronic ischemic cardiomyopathy, and two had effort angina pectoris. Dilated cardiomyopathy was diagnosed by chest X-ray (cardiothoracic ratio greater than 0.5) and echocardiogram (end-diastolic left-ventricular diameter greater than 56 mm). All of the patients underwent two successive 24-hr Holter monitoring at the time of admission and after 3, 5, and 10 days from the beginning of therapy. Ventricular arrhythmias were classified according to modified Lown criteria. Renal function was normal. Magnesium sulfate in 0.9% sodium chloride was given by slow infusions (50 mg/min/60 min) twice daily for 7 days. They were antiarrhythmic in all of the patients: VPCs and couplets mean values decreased from 7971 +/- 2612 to 321 +/- 141 (p less than 0.001) and from 405 +/- 113 to 7 +/- 4 (p less than 0.001), respectively; VT runs (33.8 +/- 5.8) disappeared by the fifth day of treatment. Both the heart rate and the QTc interval remained unchanged from baseline values. The slow magnesium infusion did not notably raise serum Mg when evaluated immediately after stopping the infusion, as compared with baseline values.(ABSTRACT TRUNCATED AT 250 WORDS)