After approval of research board of Emam Khomeini hospital and ethics committee of Urmia University Of Medical Sciences, with the aim of evaluation of intravenous ondansetron effect on hypotension during cesarean section, a randomized double-blind clinical trial will be conducted. After obtaining informed written consent 70 parturients aged at 20-40 years old with American Society of Anesthesiologists (ASA) class I-II scheduled for the elective cesarean section with spinal anesthesia will be asked to participate in the study. The patients with the history of hypertension, antihypertension drugs consumption, gastrointestinal diseases, motion sickness, allergy to ondansetron, glaucoma, preeclampsia, eclampsia, psychotic disorders and antiemetic drugs use during last 24 hours will be excluded from the study. Using random allocation software, the patients will be divided into study (O) and control (S) groups with equal numbers. In operation room, standard monitoring including noninvasive blood pressure, pulse oximetry and EKG will be attached. 5 minutes before spinal anesthesia, study and control group patients will receive 6 milligram intravenous ondansetron and normal saline solution with equal volumes respectively The patients will receive 5 ml/Kg Ringer solution intravenously and then spinal anesthesia using 12.5 mg hyperbaric bupivacaine will be induced at sitting position and at L3-L4 intervertebral level. After injecting the local anesthetic in subarachnoid space, the patient will be turned back to supine position and the operating table will be tilted 10-15 degrees to prevent aortocaval compression induced hypotension. Oxygen will be administered to the patients during surgery using face- mask. Blood pressures will be measured and recorded at minutes, 0,3,6,9,10,15,20,25,30 and 5-10 milligram intravenous ephedrine will be administered if systolic pressure decreases more than 25 percent of basic systolic or under 100 mm Hg. Mean values of blood pressure, heart rate and also nausea, vomiting and shivering during surgery and stay at recovery unit will be recorded and analyzed.