PURPOSE To assess whether intravenous magnesium sulfate infusion affects analgesic requirements during monitored anesthesia care (MAC) for shockwave lithotripsy (SWL). PATIENTS AND METHODS Fifty patients in ASA class I or II undergoing SWL with MAC were randomized into two groups. Induction of MAC was produced by bolus doses of midazolam 0.03 mg/kg(-1) and fentanyl 0.5 microg/kg(-1) followed by intravenous infusion of midazolam 0.015 to 0.06 mg/kg(-1)/hr(-1) with fentanyl supplementation 0.2 microg/kg(-1). In addition, group I patients received magnesium sulfate 30 mg/kg(-1) intravenously as a bolus dose followed by a continuous infusion of 10 mg/kg(-1) /hr(-1) beginning 15 minutes before induction of sedation. The midazolam infusion rate and additional fentanyl doses were adjusted by verbal analog scale (VAS) measurements, observer assessment sedation score (OAA/S), and achieving a target bispectral index (BIS) in a range of 70 to 90. At the end of SWL, the total midazolam and total fentanyl consumptions were recorded, and the serum magnesium concentration was measured. RESULTS Magnesium caused significant decreases in the total consumption of midazolam (P = 0.001) and fentanyl (P = 0.001). The VAS values at the 15th, 20th, and 25th minute in group I were significantly lower than in group II. In group I, hemodynamic and arterial oxygen parameters were better than in group II. CONCLUSION A magnesium bolus and infusion can be utilized to reduce analgesic requirements under MAC during SWL.