Aim. The aim of this study was to investigate the effect of i.v. infusion of magnesium sulphate during spinal anesthesia on duration of spinal block and postoperative pain. Methods. Forty ASA physical status I and status II, aged between 18 and 65, female patients undergoing abdominal hysterectomy under spinal anesthesia were enrolled in this study. Patients in the magnesium group (Group M, n = 20) received magnesium sulphate 65mg kg(-1) infusion in 250 mL 5% dextrose at 3.5 mL/min rate, and control group (Group C, n = 20) received at the same volume of saline during operation in a double-blind randomized manner. Duration of sensory and motor block, systolic, diastolic, and mean arterial blood pressures, heart rates, pain scores (VAS values), and side effects were recorded for each patient. Blood and CSF samples were taken for analysis of magnesium concentrations. Results. Regression of sensorial block was longer in Group M when compared with that in Group C (175 +/- 39 versus 136 +/- 32 min) (P < 0.01). The VAS scores were lower in Group M than those in Group C at the 2 time points postoperatively (P < 0.01). Conclusion. 65 mg kg(-1) of magnesium sulphate i.v. infusion under spinal anesthesia prolongs spinal sensorial block duration and decreases pain VAS scores without complication in patients undergoing abdominal hysterectomy.