Seven of 48 patients (14.6%) with acute myocardial infarction who were given 2.4 g of magnesium sulfate as a single intravenous dose had potentially lethal arrhythmias during the first 24 hours after admission, whereas 16 (34.8%) of 46 patients receiving placebo had similar arrhythmias. In addition, 14 of these 16 patients in the placebo group had their first arrhythmia (in the intensive coronary-care unit) within two hours after the start of the study, whereas in the magnesium-treated group, there were no such arrhythmias until some four hours later. The higher the lymphocyte potassium concentration, the greater the reduction in the incidence of arrhythmias. Serum magnesium levels increased by 16.5% and lymphocyte magnesium concentrations by 72% in the magnesium treated group. Intravenous magnesium reduces the incidence of serious arrhythmias after acute myocardial infarction.