Adequate analgesia following caesarean section decreases morbidity, hastens ambulation, improves patient outcome and facilitates care of the newborn. Intrathecal magnesium, an NMDA antagonist, has been shown to prolong analgesia without significant side effects in healthy parturients. We therefore studied the effect of adding intrathecal magnesium sulphate to bupivacaine-fentanyl spinal anaesthesia in patients with mild preeclampsia undergoing caesarean section. Sixty women with mild preeclampsia undergoing caesarean section were included in a prospective, double blind, placebo-controlled trial. Patients were randomly assigned to receive spinal anaesthesia with 2 mL 0.5% hyperbaric bupivacaine and 25 microg fentanyl with either 0.1 mL of 0.9% sodium chloride (control group) or 0.1 mL of 50% magnesium sulphate (50 mg) (magnesium group). Onset, duration and recovery of sensory and motor block, time to maximum sensory block, duration of spinal anaesthesia and postoperative analgesia requirements were studied. The onset of both sensory and motor block was slower in the magnesium group. The duration of spinal anaesthesia (229.3 vs. 187.7 min) and motor block (200 vs. 175.3 min) were significantly longer in the magnesium group. Diclofenac requirement for 24 h following surgery was significantly lower in the magnesium group (147.5 vs.182.5 mg, P=0.02). Haemodynamic parameters and side effect profile were similar in the two groups. In parturients with mild preeclampsia undergoing caesarean delivery, the addition of magnesium sulphate 50 mg to the intrathecal combination of bupivacaine and fentanyl prolongs the duration of analgesia and reduces postoperative analgesic requirements without additional side effects. Copyright 2009 Elsevier Ltd. All rights reserved.