Background: Intrathecal adjuvants are used for prolongation of duration of subarachnoid block, and provide adequate analgesia. Aim: To evaluate the onset of sensory and motor block, level of sensory block, highest sensory level and Bromage grade at time of onset, induced by Dexmedetomidine and Magnesium sulphate when given intrathecally with 0.5% hyperbaric Bupivacaine for spinal anaesthesia. Materials and methods: 90 patients, of ASA grade I and grade II, age 18-65 years, of either gender, height 150 cm and above and weight 50 kg to 80 kg, scheduled for lower abdominal and lower limb surgery under spinal anaesthesia. They were randomly assigned according to table of randomization into three groups, 30 patients in each group. Patients in Group A were given 15 mg hyperbaric Bupivacaine and 10 μg (0.1 ml) Dexmedetomidine and those in Group B were given 15 mg hyperbaric Bupivacaine and 50 mg (0.1 ml) magnesium sulphate. Patients in Group C were given 15 mg hyperbaric Bupivacaine and normal saline (0.1 ml) as control. The onset of sensory block, motor block, level of sensory and grade of motor block were recorded at the time of onset. Results: Onset of sensory and motor block was shorter in group Dexmed as compared to other two groups, and prolonged in group magnesium as compared to other two groups. The level of sensory block was higher at the time of onset and grade of motor block was more in group Dexmed than other two groups. Conclusion: Addition of Dexmedetomidine to Bupivacaine shortens the onset time whereas addition of Magnesium delays onset.