BACKGROUND: Mg++ is an important control factor for transport of K+, Na+, Ca++ and also has been known for having an antiarrhythmic and inotropic action on the heart. Orthotopic liver transplantation is a complex surgical procedure with significant physiologic alterations resulting in electrolyte imbalances. Therefore, the goal of this study was to evaluate the effects of prophylactic MgSO4 administration on intraoperative serum magnesium concentration and its related clinical outcomes. METHODS: 19 patients receiving liver transplants were divided into two groups. Both groups received normal saline (300 ml/3 hr) right after anesthetic induction. MgSO4 (35 mg/kg) was mixed with saline in the experimental group but not in the control group. Serum magnesium concentration was measured four times during surgery. We also checked the total transfused units of packed cells and CaCl2 requirement, the severity of postreperfusion syndrome and base deficit. RESULTS: Serum magnesium concentration significantly decreased in the control group at the postanhepatic stage. The experimental group showed less prominent symptoms of postreperfusion syndrome and less need for potassium supply, but both groups did not reveal any differences in the amount of transfusion and CaCl2 requirement. CONCLUSIONS: We confirmed that our prophylactic administration of MgSO4 (35 mg/kg) may be considered a safe dose showing not only prevention of hypomagnesemia but also a decrease in the potassium requirement and in the severity of postreperfusion syndrome. However, routine administration of MgSO4 might cause disadvantageous effects, so more appropriate indications can be prepared after further clinical research.