Introduction Fundamental of regional anesthesia is pharmacologically interrupting transmission of sensation in the specific nerve fiber. The sensory signals generated by tissue damage triggers a state of increased excitability, leading to prolonged post-operative pain or sensitization to such pain. The optimal pain treatment pre-empts the establishment of pain hypersensitivity during and after surgery by minimizing the patient discomfort while leaving physiologic nociceptive mechanisms intact so as to function as an early warning symptom. Aim To measure the effect of intravenous magnesium infusion during spinal anesthesia on post operative analgesia in patients undergoing total hip replacement. Patients and Methods Patients were randomly assigned to two groups using closed envelopes chosen by patients before the study. Patients in the magnesium group (Group M, n = 15) received magnesium sulphate 50 mg per Kg over 15 min after spinal anesthesia and then 15 mg per Kg per hr by continuous i.v. infusion until the end of surgery. Patients in the saline group (Group S, n = 15) received the same volume of isotonic saline over the same period. Infusions were prepared in pharmacy and they were administered using the infusion machines. Study data were recorded by an observer unaware of group assignments. Results the study showed that intravenous magnesium infusion during spinal anesthesia decreased VAS results in magnesium group specifically in the first 24 hours and hence decreasing need for post operative rescue analgesic consumption .Also post operative nausea,vomiting and shivering were lower in magnesium group. Conclusion We concluded that intravenous magnesium infusion during spinal anesthesia improves post operative analgesia and reduces incidence of nausea, vomiting and shivering in patients undergoing total hip replacement.