Background and Aims: Hemodynamic instability is most common intraoperative complication after spinal anesthesia during cesarean delivery. Spinal anesthesia causes bradycardia and hypotension via activation of Bezold-Jarisch reflex. Ondansetron is specific 5-HT3 receptor antagonist that alleviates the Bezold-Jarisch reflex lead to decrease hypotension and bradycardia. The present study was done to compare the effect of intravenous ondansetron with placebo for attenuation of spinal induced hypotension, change in heart rate, requirement for vasopressor and incidence of shivering and postoperative nausea- vomiting. Material and Methods: 120 Patients of ASA1/2, age 20-35years, weight 40-60Kgs undergoing cesarean delivery under spinal anesthesia were selected for study. History of PIH, convulsion or allergy to the drug used, requires general anaesthesia for supplementation were excluded from study. Randomization was done by chit in box method. Group O (n=60) received 6mg ondansetron in normal saline intravenously; total volume made 10ml. Group S (n=60) received 10ml normal saline intravenously. Blood pressure and heart rate were checked every 5minutes till the end of the surgery. Data was analyzed by chi square test. Results: Systolic, diastolic and mean blood pressure were found to be higher in group O as compare to group S at different time intervals (P value < 0.05). In group O 67% patients required vasopressors whereas in group S 91% patients required vasopressors. Incidence of nausea and vomiting is less in group O (P value=0.001). Conclusion: Prophylactic intravenous ondansetron causes reduced incidence of hypotension, requirement of vasopressors and Post-operative nausea-vomiting during spinal anaesthesia.