Magnesium is of great importance in cardiac arrhythmias. It increases the ventricular threshold for fibrillation. Sinus node refractoriness and conduction in the AV node are both prolonged. Main indications for intravenous application of magnesium are Torsade de pointes tachycardias, digitalis toxicity induced tachyarrhythmias and multifocal atrial tachycardias. Additionally, patients with ventricular arrhythmias due to overdoses of neuroleptics or tricyclic antidepressants may profit from i.v. magnesium. Monomorphic ventricular tachycardias and ventricular arrhythmias refractory to class III antiarrhythmics have been shown to respond to i.v. magnesium. Recent publications have documented that perioperative use of magnesium can reduce the incidence of arrhythmic events on the atrial and ventricular level. Oral magnesium has been used for many years in patients with symptomatic extrasystoles. Studies show that the incidence of extrasystoles as well as patients' symptoms are reduced during oral magnesium therapy.