Magnesium is an essential transmembrane and intracellular modulator of the electrical activity of cardiac cells. This review provides an up-to-date consideration of the cellular and clinical electrophysiological role of magnesium. This ubiquitous element seems to be important from both the theoretical and clinical point of view, because magnesium salts (MgSO4, MgCl2) administered intravenously are particularly effective in those arrhythmias in which the mechanism involves early or delayed after depolarization-induced triggered activity. The authors share the view that I.V. magnesium is the drug of choice in "torsade de pointes" ventricular tachycardia accompanying acquired long QT/QTU syndrome. It is complementary therapeutic agent in digitalis-induced tachycardias. Further studies are needed to elucidate magnesium's mode of action and efficacy in other types of clinical tachyarrhythmias.