BACKGROUND: Epidural buprenorphine provides good pain relief after Cesarean section, but is often associated with nausea and vomiting. Ondansetron, a selective 5-hydroxytryptamine 3 (5-HT3) receptor antagonist, is known to prevent and treat emesis after chemotherapy in cancer patients and after general anesthesia. The purpose of this study was to compare the prophylactic antiemetic effect of ondansetron and metoclopramide on nausea and vomiting after epidural buprenorphine. METHODS: Sixty women undergoing Cesarean section were studied. The patients were given subarachnoid injections of 0.5% tetracaine 9 mg and were inserted with epidural catheters for postoperative pain control. Prior to closure of the peritoneum, we injected a mixture of buprenorphine and bupivacaine through the epidural catheters and gave intravenous boluses of saline 6 ml, metoclopramide 10 mg and ondansetron 4 mg randomly. The incidence of nausea and vomiting and the degree of satisfaction were evaluated until 24 hr after the injection of epidural buprenorphine. RESULTS: The number of patients who became nauseated or vomited did not differ significantly between the ondansetron group and the metoclopramide group. Also, subjective ratings of satisfaction and incidence of other side effects did not differ significantly between the groups. CONCLUSIONS: Ondansetron, administered intravenously, prevented postoperative nausea and vomiting associated with epidural buprenorphine equally as well as metoclopramide.