Life-threatening cardiac arrhythmia is a frequent complication of open heart surgery. There are many causes of postoperative cardiac arrhythmias. Electrolyte imbalance such as hypokalemia and acidemia are major causes of ventricular arrhythmias. Infrequently, however, antiarrhythmic agents and /or hypomagnesemia induce[s] a ventricular arrhythmia such as "torsade de pointes" by increasing the repolarization time of myocardium, Recently, we have experienced two cases of "Torsade de pointes" associated with hyp-omagnesemia after replacement of mitral valve and one of whom after use of procainamide. Intravenous infusion of magnesium immediately and successfully abolished the torsade de pointes in both cases.intes in both cases.