The purpose of this study is to compare the administration of two different doses of ondansetron to placebo to prevent hypotension and bradycardia following spinal anaesthesia. Apart from haemodynamic parameters (blood pressure and heart rate),characters of the spinal blockage (time of onset and regression) will be recorded too.

The day before surgery the procedure will be explained to the patient and the written consent will be obtained. In the operating room, intraoperative monitoring will include electrocardiography (ECG), noninvasive blood pressure, oxygen saturation by pulse oximetry (SpO2) and heart rate (HR). Two peripheral intravenous catheters wil be placed for fluid replacement and administration of drugs.

Participants will be randomly assigned to one of the following groups:

Group A: Women will receive 4 milligrams (mg) of ondansetron diluted in 100 milliliters (ml) of normal saline 0.9 percent 10 minutes before spinal anaesthesia

Group B:Women will receive 8 mg of ondansetron diluted in 100ml of normal saline 0.9 percent 10 minutes before spinal anaesthesia

Group C:Women will receive 100ml of normal saline 0.9 percent 10 minutes before spinal anaesthesia

Subsequently, after receiving 500ml of colloid solution, spinal anesthesia will be performed at level L3-L4 or L4-L5 in the vertebral space with 1.6ml of 0.75 percent ropivacaine and 15mcg of fentanyl, using a 27-gauge pencil point spinal needle with patients in a left lateral position. After subarachnoid infusion, participants will be placed supine with left uterine displacement and anesthetic and motor blockage will be evaluated every one minute until anesthetic blockage reaches the level of T4 neurotome and the motor block becomes complete (Bromage grade 3). This time will be called Time to max effect (Tmax).

Hypotension, defined as systolic blood pressure below 100 millimeters of Mercury (mmHg), will be treated using 5mg ephedrine if the heart rate is less than 100 beats per minute or with 20mcg of phenylephrine if the heart rate is greater than 100 beats per minute. Bradycardia, defined as a fall in heart rate below 60 beats per minute will be treated with atropine (0.6mg).

Immediately after the delivery of the neonate, all women will receive a solution of oxytocin (20 units) intravenously. Half an hour before the end of the procedure they will receive an additional 1g of paracetamol and 75mg of diclofenac.

Inclusion Criteria:

• Physical status according to American Society of Anesthesiologists (ASA) I-II
• Singleton pregnant women in full term pregnancy
• Patients scheduled for cesarean section
• Height 158cm-170cm

Exclusion Criteria:

• patient's own refusal
• contraindications to spinal anesthesia (coagulation disorders, inflammation at the puncture site, allergy to local anesthetics)
• ondansetron allergy
• body mass index> 33kg / m^2
• height <158cm, or> 170cm
• hypertensive disorders of pregnancy
• cardiovascular disease
• receiving selective serotonin reuptake inhibitors (SSRI's) or treatment for migraine
• placenta previa.

All solutions will be prepared by an independent researcher who will not be further involved in the study eg data collecting or analyzing them. All solutions will look identical to the anesthetist who will administer them to the patients.Apart from the anesthetist, the surgery staff and the researchers recording the measurements will not know the therapeutic intervention team in which each patient has been randomized.