Study Objective: To compare the relative efficacy of prophylactic metoclopramide, ondansetron, and Placebo in nonemergent cesarean section patients given epidural anesthesia intraoperatively and for the first 24-hour period after delivery. Design: Randomized, double blind, placebo-controlled study. Setting: Inpatient obstetric unit at a university hospital center. Patients: 164 nonemergent cesarean section patients given epidural anesthesia. Intervention: At time of umbilical cord clamp, patients received intravenously (IV) either 4 mg ondansetron (Group O) or 10 mg metoclopramide (Group M) or 10 mL normal saline (Group P). Measurements and Main Results: Episodes and severity of nausea and vomiting, rescue antiemetic requirement, Patient satisfaction, and side effects were recorded. The frequency of intraoperative nausea were 24%, 43%, and 57% for Group O, Group M, and Group P, respectively (p < 0.03). The frequency of nausea for the 24-hour study period were 26%, 51% and 71% for Groups O, M, and P respectively (p < 0.03). The frequency of intraoperative and postoperative vomiting were similar between Group 0 and Group M, but significantly higher in Group P (p < 0.05). Overall patient satisfaction was highest in Group O compared with Groups P and M (p < 0.05). Maximum analog sedation score was higher in Group M compared to Groups O and P (p < 0.05). Conclusions: In cesarean section Patients given epidural anesthesia, prophylactic ondansetron, 4 mg TV, is more efficacious and has a higher patient satisfaction than that with metoclopramide, 10 mg IV, or placebo in preventing nausea and achieving complete responses during intraoperative period and the first 24-hour postdelivery period. However, there is no difference between ondansetron and metoclopramide in reducing-frequency of vomiting. Prophylactic ondansetron 4 mg TV is more effective in preventing nausea than vomiting. (C) 2001 by Elsevier Science Inc.