To investigate the efficacy of the preemptive analgesic effect of low dose ketamine versus MgSO 4 on potentiating postoperative analgesia when compared to placebo in patients undergoing cesarean section under general anesthesia. This prospective randomized blind study included 60 parturient females scheduled for cesarean section under general anesthesia. They were randomized into three equal groups [20 patients each]: group I [Control]: received 20ml normal saline 0.9%, group II [Ketamine]: received ketamine 0.3mg/kg in 20ml normal saline and group III [MgSO 4 ]: received MgSO 4 , 30mg/kg in 20ml normal saline. Investigated medications infused over 10min before induction of anesthesia. Fentanyl requirement, mean BP and HR were measured intraoperatively. Pain, sedation and nausea and vomiting were assessed at 2, 6, 12 and 24h postoperatively. Total postoperative pethidine dose over 24h was calculated. MBP and HR showed statistically significantly lower value in group (K) and (M) compared to group (C) at postintubation and postincision readings. Intraoperative fentanyl requirement was statistically higher in (C) groups compared to (K) and (M) groups. The time for first request for postoperative analgesia was longer and the total postoperative pethidine dose over 24h was lower in (K) group compared to (C) and (M) groups. Postoperative VAS scores at 2 and 6h postoperatively showed statistically highly significantly lower values in (K) group compared to (C) and (M) groups. Apgar, postoperative sedation and PONV scores showed no statistically significant differences among the three groups. Preemptive dose of either ketamine (0.3mg/kg) or MgSO 4 (30mg/kg) in patients undergoing cesarean section under general anesthesia could suppress the pressor response to endotracheal intubation and skin incision and decreased the intraoperative fentanyl requirement. Ketamine showed a significant preemptive analgesic effect compared to MgSO 4 at 2 and 6h postoperatively.