Background : This study was designed to evaluate the continuous effects of single intravenous injection of antiemetics on nausea and vomiting during continuous morphine injection for postoperative pain control. Methods : Prior to the study, we divided patients into two major groups according to the type of surgery performed intra-abdominal(Open: O) and non intra-abdominal(Close: C). When patients regained orientation after routine general anesthesia, enflurane-$O_2-N_2O$, we injected bolus dose of morphine and started continuous injection of morphine for postoperative pain control(Group I; Control). After bolus injection and just before continuous injection, we injected single dose of droperidol(Group II) or ondansetron(Group III). Mean arterial blood pressure, heart rate, pain score and symptom-therapy score were checked at 10 minutes, 4, 8, 16, 24, 36 hours after continuous morphine injection. Results : The pain score of group III was lower than group II(10 min.) and group I(24, 36 hours) in the open group. Symptom-therapy score of group III(10 min., 4, 24 hours) and group II(10 min.) were lower than group I in the open group. In the close group, symptom-therapy score of group III(8 hours) was lower than group I. Conclusions : Single intravenous injection of antiemetics have a tendency of lowering symptom-therapy score for 36 hours in spite of their relatively short elimination half-life.