Context: Magnesium prolongs analgesia in humans when given intrathecally. Fentanyl being highly lipid soluble diffuses into spinal cord and binds to dorsal horn receptors rapidly when administered intrathecally.Aims: To compare the effects of magnesium sulphate and fentanyl with bupivacaine for post operative analgesia in perianal surgeries. Material and methods: 42 patients aged 18-65 years of ASA Grade I and II scheduled for perianal elective surgeries under spinal anaesthesia were allocated equally in two groups according to chit system randomly. Group BF- Inj. bupivacaine 0.5% 1 ml+25 µg fentanyl (preservative free) 0.5 ml. Group BM- Inj. bupivacaine 0.5% 1 ml+100 mg magnesium sulfate (preservative free) diluted to 0.5 ml with normal saline. Patients were observed for onset and duration of sensory and motor blockade, duration of postoperative analgesia and analgesic requirement in first 24 hours. Statistical analysis: The statistical analysis was assessed by unpaired t-test on Microsoft excel and IBM SPSS version 21. Results: Duration of sensory block, motor block and postoperative analgesia was significantly more in BM group than in BF group (p<0.001). Time to rescue analgesia in group BF was significantly less than that in group BM (p<0.001). Conclusion: Though the onset of motor and sensory block is delayed with intrathecal magnesium sulfate, duration of analgesia is prolonged as compared to intrathecal fentanyl. Total analgesic requirement is significantly more with fentanyl as compared to magnesium.