Background: Subarachnoid block is commonly used technique for anaesthesia in different surgical specialties. Ondansetron is a selective 5-HT 3 antagonist. It is an effective drug for prevention and treatment of nausea and vomiting, so being used generously as antiemetic drug in premedication. Any interaction with regional anaesthesia including the effect on sensory and motor block would be particular interest for anaesthesiologist. This study was done to compare the effect of intravenous ondansetron on sensory and motor blockade induced by intrathecal spinal hyperbaric bupivacaine in patients undergoing orthopaedic surgery. Methods: 60 unpremedicated patients of male sex, age 20- 45 years belonging to an ASA physical status I or II is undergoing lower limb orthopaedic procedures being performed under intrathecal spinal anaesthesia were recruited for this study. Group C received i.v saline 2ml and Group O received i.v Ondensetron 4 mg (2ml) 15 min prior to the intrathecal spinal hyperbaric bupivacaine. Sensory and motor block level was assessed in this study. Results: We found that in patients group C (control) had the highest block level T4 as compare to group O (ondansetron pretreatment group) in which the highest sensory block level was T6. For maximum sensory block level comparison between group C and group O found significant (p<0.001). Duration of the block is also decreased in ondansetron group, i.e. those who had received ondansetron prior to spinal anaesthesia. Conclusion: It was observed in our study that the level of maximum sensory block height was significantly lower in ondansetron pretreatment group for hyperbaric bupivacaine. It was also noted that the ondansetron pretreatment group had faster regression of sensory block level. No significant differences were detected between the study groups for motor block.