We evaluated the antiarrhythmic response to magnesium given intravenously to 40 patients who had had elective cardiopulmonary bypass for the surgical treatment of acquired cardiac disease. All 40 patients were found to have hypomagnesemia postoperatively as defined by a serum magnesium level of less than 1.8 mg/dL. The ventricular arrhythmias of these patients were categorized by a modified Lown grade classification. Magnesium sulfate (16 mEq, [2000 mg]) was administered, and the patients were reclassified according to their rate of ventricular ectopia. All arrhythmias of a higher grade responded to the treatment with a marked reduction in ventricular ectopia and were reclassified to a lower grade. This study supports the routine administration of magnesium sulfate, at a dose of 16 mEq, in postoperative cardiac patients who are hypomagnesemic.