We report a case of the accidental intravenous administration of a large dose of magnesium sulfate during cesarean section. A 41-year-old woman, at 33 weeks gestation, with pregnancy-aggravated hypertension, headache and generalized edema presented in acute labor and showed fetal bradycardia on a nonstress test. Laboratory tests demonstrated an increased level of magnesium (5.4 mg/dl). A cesarean section was performed under general anesthesia with O2-N2O-enflurane and vecuronium. After delivery 2,000 mg of magnesium sulfate was mixed with the lactated Ringer's solution 1,000 ml and 550 ml administered to the patient. After noticing the accidental infusion of the magnesium sulfate, we replaced the lactated Ringer's solution with normal saline 1,000 ml and performed arterial blood gas analysis, checked serum electrolyte, including Mg2+ and Ca2+, and had monitored depth of muscle relaxation and vital signs. The level of magnesium had increased to 8.9 mg/dl after the accidental magnesium infusion. For about one and half hours after emergence from general anesthesia, she complained of dyspnea and paraparesis of extremities. To treat the hypermagnesemia, 3% calcium gluconate 1,000 mg and furocemide were given intravenously to antagonize magnesium and to increase the urine output. The depth of neuromuscular block was frequently monitored using a nerve stimulator. After conservative treatment, she recovered from the effect of the hypermagnesemia and was discharged on the fifth postoperative day. Anesthesiologists must to keep in mind the preoperative patients' pathophysiologic conditions, check co-administered drugs and the contents of intravenously connected solutions.