Introduction: Spinal anaesthesia is now the effective technique for caesarean section, but hypotension is the main complication which adversely affect both mother and neonate.

Aim: To assess the efficacy and safety of prophylactic dose of ondansetron in comparison to phenylephrine to prevent spinal anesthesia– induced hypotension   in caesarean section.

Materials and Methods: This prospective, randomized, parallel, double–blinded, placebo controlled unicentric clinical trial was conducted on 63 pregnant mothers who fulfilled the subject selection criterias.Patients were allocated into three groups: Group P received 100 mcg of phenylephrineintravenously, Group O received 4 mg of IV ondansetronintravenously and Group C received 10 ml of normal saline intravenously. Systolic blood pressure (SBP),diastolic blood pressure (DBP) ,mean blood pressure(MBP), heart rate (HR) were recorded in three (3) minutes interval during caesarean section and every ten (10) minutes interval after caesarean section for one hour.Adverse effects,if any were also noted during the study time.Chi–square test, Kruskall–Wallis (nonparametric) test, Friedman’s ANOVA followed by Dunn’s Multiple Comparison were performed by Graph Pad InStat 3 software for statistical analysis.

Result : Patients of Group P had least episode than other groups.Incidence of hypertensive episode was maximum in group C, followed by group O. rescue medication was required maximum in group C than other two group.. There was no difference between group O and group P in respect of total amount of rescue medication.

Conclusion: Prophylactic 4 mg IV ondansetron is effective and safe to prevent the spinal anaesthesia induced hypotension with minimal adverse effect.