In a double-blind placebo-controlled study, 130 patients with verified acute myocardial infarction were given magnesium or placebo treatment intravenously immediately upon admission to hospital. The incidence of arrhythmias requiring treatment during the initial week of hospitalization was registered. Serum magnesium concentrations were increased from 0.7 mmol/l to 1.3 mmol/l as a result of the magnesium infusions. This pharmacologically induced hypermagnesemia resulted in a reduction in the incidence of arrhythmias from 47% in the placebo group to 21% in the magnesium group (p = 0.003). In the magnesium-treated patients, increments in serum concentrations of magnesium and potassium correlated positively (r = 0.47, p less than 0.001). It is concluded that magnesium infusion in the postinfarct period reduces the incidence of supraventricular tachyarrhythmias, and possible pathophysiological mechanisms involved are discussed.