BACKGROUND Magnesium is a noncompetitive blocker of N-Methyl-D-Aspartate (NMDA) receptor with anti-nociceptive effects. Recently it has gained popularity as an adjuvant in spinal anaesthesia. The addition of intrathecal magnesium sulphate to hyperbaric bupivacaine prolongs the duration of analgesia. In this prospective, randomized, double-blind study, we investigated the effect of adding intrathecal magnesium sulphate to bupivacaine in spinal anaesthesia for lower abdominal surgeries. MATERIALS AND METHODS Sixty ASA I or II adult patients of either sex undergoing lower abdominal surgeries were recruited. They were randomly allocated into two groups- Group M received intrathecal 15 mg bupivacaine (0.5% hyperbaric solution) combined with 0.5 mL of 10% MgSO4; Group B received 15 mg bupivacaine (0.5% hyperbaric solution) combined with 0.5 mL of normal saline. We evaluated the onset of sensory and motor block, the duration of sensory and motor blockade and duration of spinal anaesthesia. RESULTS Onset of sensory and motor block was significantly shorter in Group B (2.06±0.79, 3.03±0.65 min) than Group M (4.84±2.24, 7.38±1.14 min).The duration of sensory block was significantly higher in Group M (220.78±9.4 min) than in Group B (165.78±17.6) The mean duration for complete motor recovery was maximum in Group M (271.44±34.72 min) than in Group B (180±18.41 min). Duration of analgesia was also longer in Group M (340.62±15.4 min) than with Group B (234.6±27.14 min) which was statistically significant (P<0.05). CONCLUSION In patients undergoing lower abdominal surgery, the addition of intrathecal magnesium sulphate to spinal anaesthesia induced by bupivacaine delayed the onset of both sensory and motor blockade, but improved the quality and the duration of postoperative analgesia.