Magnesium is an important cation that has a key role in cellular processes of energy transfer and utilization involving adenosine triphosphate, and influences cell membrane functions. Its antiarrhythmic properties are well-known and it is widely recognized as an adjunct for the treatment of arrhythmias after myocardial infarction and cardiopulmonary bypass. Magnesium may influence hemodynamic performance through its effects on vascular tone, modulation of intracellular calcium, regulation of catecholamine activity, and its essential role in adenosine triphosphate metabolism. The potential for magnesium deficiency to affect cardiovascular performance may be especially relevant in ischemic states. We report a case of cardiogenic shock developing after cardiopulmonary bypass that was initially unresponsive to therapeutic intervention, but that resolved promptly after magnesium administration. The potential role of magnesium in enhancing hemodynamic performance is discussed, with a review of its cellular metabolic properties and activities.