Background: Caesarean section is one of the most common gynecologic surgeries. Nausea and vomiting after pain is the most common side effect of surgeries, today, we most widely use of serotonin receptor antagonists, that are most effective antiemetic and have less side effect rather than other drug. Ondansetron is typical serotonin antagonists.

Materials and Methods: This is a randomized, double-blind clinical trial on 162 women undergoing cesarean section. 4mg intravenous ondansetron was given to first group and 8mg oral ondansetron was given to second group and thired group recived placebo. Blood pressure and heart rate were measured before and immediately after performing spinal and every 5 minute until 20 minute. Post operative nausea and vomiting and APGAR score were recorded after operation in recovery and 2, 4, and 6 houres after surgery.

Results: There was no significant difference among 3 groups according to age, gravidity, mean atrial pressure and heart rate before and during surgery. Nausea and vomiting in oral and intravenous groups at recovery, 2 and 4 hours after surgery were significantly less nausea and vomiting than placebo group (p<0.05). Whereas there was no significant difference between oral and intravenous groups.

Conclusion: It was concluded that using oral ondansetron with the same antiemetic effect is more convience to women undergoing cesarean section with spinal anesthesia.