Objective To compare the effect of midazolam or tropisetron or midazolam- tropisetron on PONV in patients undergoing lower segment cesarean section under spinal anesthesia.Methods 160 healthy full-term parturients scheduled for elective cesarean section were randomly divided into four groups:placebo group(S,n=40),tropisetron group(T,n=40),midazolam group(M,n=40) and midazolam-tropisetron group(MT, n=40) . All patients were positioned in the right lateral decubitus and a 22 gauge spinal needle was introduced through mid-line approach at the L3-L4 inter-space. Patients received 2 ml of 0.75% ropivacaine(hypobaric) + 1ml of cerebrospinal fluid subarachnoid injection. Group S (n=40),which received 4 ml of normal saline, Group T(n=40),which received 4 ml of tropisetron (4mg), Group M (n=40), which received 4ml of midazolam(40ug/kg) and Group MT(n=40), which received midazolam (40ug /kg) + 4ml of tropisetron (4mg) after clamping of the umbilical cord. Postoperative pain management was carried with patient controlled intravenous analgesia. Then the severity of nausea was measured by visual analog scales /score (VAS). Each patient was observed and asked for at 0-6h、6-12h、12-18h、18-24h after surgery occurrence of nausea and vomiting. Results The incidence of PONV in group M（2.5%）and group MT（2.5 %）was significantly lower than that in group S (37.5 %)and group T（20%）. Group M was as equally effective as group MT for PONV. Group T was not significantly, compared with group S for PONV. The request for a rescue antiemetic of group S (20%) and group T(10%) was significantly higher ,compared with group M(0) and group M(0).Conclusion Midazolam or midazolam-troposetron combination was more effective than tropisetron or placebo group in the prevention of postoperative nausea and vomiting in lower segment cesarean section under spinal anesthesia . Midazolam alone was as equally effective as midazolam-tropisetron combination.