79 patients undergoing lower extremity orthopedic surgery were randomized to examine the effect of additional magnesium sulfate (MgSO4) to bupivacaine on spinal anesthesia. Patients received either 100 mg magnesium sulfate 5% (0.2 ml) with 15 mg bupivacaine 0.5% or 15 mg bupivacaine 0.5% plus 0.2 ml normal saline (control). Assessment after the first 24 hours revealed that the addition of magnesium sulfate to bupivacaine lengthened the duration of sensory block, reduced postoperative analgesic consumption, and significantly prolonged the onset of spinal anesthesia.