Background: Magnesium is one of the effective, safe local anesthetic adjuvants that can exert an analgesic effect in conditions presenting acute and chronic post-sternotomy pain. We studied the efficacy of continuous infusion of presternal magnesium sulfate with bupivacaine for pain relief following cardiac surgery. Methods: Ninety adult patients undergoing valve replacement cardiac surgery randomly allocated into three groups. In all patients; a presternal catheter was placed for continuous infusion of either 0.125% bupivacaine and 5% magnesium sulfate (3 ml/h for 48 hours) in group 1, or 0.125% bupivacaine only in the same rate in group 2, versus conventional intravenous paracetamol and ketorolac in group 3. Rescue analgesia was iv 25 g fentanyl. Postoperative Visual Analog Scale (VAS) and fentanyl consumption during the early two postoperative days were assessed. All patients were followed up over two months for occurrence of chronic post-sternotomy pain. Results: VAS values showed high significant differences during the first 48 hours with the least pain scale in group 1 and significantly least fentanyl consumption (30.8 ± 7 g in group 1 vs. 69 ± 18 g in group 2, and 162 ± 3 in group 3 respectively). The incidence of chronic pain has not differed between the three groups although it was more pronounced in group 3. Conclusions: Continuous presternal bupivacaine and magnesium infusion resulted in better postoperative analgesia than both presternal bupivacaine alone or conventional analgesic groups.