Abstract： Background Perioperative disorder of magnesium,an important cation in the human body,may affect clinical anesthesia. The pharmacological data of propofol use,which is popularly used in the anesthesiology department and intensive care unit,is incomplete in Chinese patients. This study aimed to assess the effect of magnesium sulfate on the calculated plasma medial effective concentration (Cp50cal) of propofol at loss of response to command in Chinese females.Methods Fifty patients undergoing gynecological laparoscopic surgery were randomly divided into the control group and the magnesium group. Before induction,magnesium sulfate (30 mg/kg) or placebo (equal volume of 0.9% saline)was given to patients in the magnesium group or the control group,respectively. Propofol was infused using a target-controlled infusion system,with a target concentration for each patient decided by up-and-down sequential allocation. After the equilibration between target plasma concentration and effective-site concentration,the state of consciousness was assessed. For determination of serum magnesium and calcium concentration,blood samples were taken before induction,after induction and at the end of surgery.Results The Cp50cal was 2.52 μg/ml (2.47-2.57 μg/ml) for patients in the control group,and 2.46 μg/ml (2.41-2.51μg/ml) for those in the magnesium group. A significant reduction of Cp50cal was observed (P=0.021). There was a significant difference between the serum magnesium concentrations after induction and at the end of the surgery (P ＜0.05). In the magnesium group,there was a trend toward decreasing after surgery,while in the control group,Cp50cal decreased significantly (P ＜0.01).Conclusions Cp50cal of propofol at loss of response to command was 2.52 μg/ml (2.47-2.57 μg/ml) for Chinese female adults in this study. Infusion of 30 mg/kg magnesium sulfate may reduce the Cp50cal of propofol at loss of consciousness,which implies that magnesium may enhance the pharmacological effects of propofol.