BACKGROUND: Recent studies suggested that preoperative block of N-methyl-D-aspartate (NMDA) receptor with NMDA antagonists may reduce postoperative pain. In this double-blind study, we administered magnesium sulfate, a natural NMDA receptor antagonist, to investigate the preemptive effect of magnesium sulfate on postoperative pain. METHODS: Seventy-three patients scheduled for gastrectomy were randomly assigned to one of three groups. Groups 2 and 3 received intravenous magnesium sulfate (Group 2: 50 mg/kg bolus, 7.5 mg/kg/h for 20 h, preoperatively, Group 3: 50 mg/kg bolus, 7.5 mg/kg/h for 20 h, postoperatively). Group 1 received normal saline as the control group. Visual analog scale (VAS) for postoperative pain and mood, cumulative analgesic comsumption, recovery of pulmonary function and side effects were evaluated at 6, 24, 48 and 72 hours after the operation. RESULTS: In groups 2 and 3, plasma concentration of magnesium were significantly higher than group 1 after 6 and 20 hours after infusion. There were no significant differences in the pain, mood, cumulative analgesic comsumption, and recovery of pulmonary function and the incidence of side effects at 6, 24, 48 and 72 hours after the operation among the three groups. CONCLUSIONS: We conclude that intravenous infusion of magnesium is not effective in reducing postoperative pain.