Several small, double-blind, randomized studies have shown that infusion of magnesium in patients with acute myocardial infarction reduces incidence of arrhythmias and mortality quite considerably. These beneficial effects are found in elderly patients too. Compared with placebo, magnesium infusion seems to increase myocardial salvage and protect against evolution of infarct. The mode of action of magnesium in reducing arrhythmias and mortality in acute myocardial infarction has not been clarified. Several factors may be involved. It has been shown that the antiarrhythmic effect of magnesium is related to a rapid rise in serum concentration. Magnesium possesses calcium-antagonistic properties, and reduces tone in smooth muscle cells. The use of magnesium decreases coronary artery spasm and total peripheral resistance, and increases cardiac output. Beneficial effects on blood platelets have also been reported. Magnesium therapy is cheap and easy to administer; contradictions include reduced renal function, disturbances in atrioventricular conduction and hypotension.