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The authors investigated whether an intravenous administration of magnesium sulfate reduces propofol infusion requirements during maintenance of propofol–N2O anesthesia.Part I study: 54 patients undergoing total abdominal hysterectomy were randomly divided into two groups (n = 27 per group). The patients in the control group received 0.9% sodium chloride solution, whereas the patients in the magnesium group received magnesium (50 mg/kg as a bolus, then 8 mg · kg−1 · h−1). To maintain mean arterial blood pressure (MAP) and heart rate (HR) at baseline value, the propofol infusion rate was changed when the MAP or the HR changed. The amount of propofol infused excluding the bolus dosage was divided by patient's body weight and total infusion time. Part II study: Another 20 patients were randomly divided into two groups (n = 10 per group). When the MAP and HR had been maintained at baseline value and the propofol infusion rate had been maintained at 80 μg · kg−1 · min−1 (magnesium group) and 160 μg · kg−1 · min−1 (control group), bispectral index (BIS) values were measured.Part I: The mean propofol infusion rate in the magnesium group (81.81 ± 13.09 μg · kg−1 · min−1) was significantly less than in the control group (167.57 ± 47.27). Part II: BIS values in the control group (40.70 ± 3.89) were significantly less than those in the magnesium group (57.80 ± 7.32).Intravenous administration of magnesium sulfate reduces propofol infusion requirements. These results suggest that magnesium administration may have an effect on anesthesia or analgesia and may be a useful adjunct to propofol anesthesia.