The aim of the study is to study the effect of preoperative intravenous Ondansetron on spinal induced hypotension in obstetric and non-obstetric patients. Primary objective of this prospective randomized double blind study is to find whether intravenous Ondansetron given prophylactically prevent spinal anaesthesia induced hypotension. Secondary objective is to observe effect of intravenous Ondansetron given prophylactically on pulse, nausea, vomiting and shivering. An observational randomized double blind study was done from 12/2018 to 9/2019 in tertiary care hospital of western Maharashtra after approval of institutional ethical committee (IEC/406). 122 patients ASA –I & ASA-II in age group of 18years to 60 years scheduled to undergo obstetric (caesarean section) and non-obstetric (infraumblical) surgeries under spinal anaestheisa were randomly divided into 4 groups. The study concluded that 4 mg Ondansetron when given intravenous prevent spinal induced hypotension in obstretic patients for caesarean section as well as in patients undergoing infraumblical surgeries which are most common surgeries performed in indian scenario.