Postoperative emesis is still a troublesome problem in patients undergoing general anesthesia. We compared the efficacy and safety of ondansetron, a selective antagonist of serotonin 5HT3 receptors, within the 24hours postoperative period. Forty patients between the ages of 12 and 75 and ASA class I-III, having general anesthesia, undergoing elective abdominal surgery (except gynecologic surgery) were assigned to one of two treatment groups according to uncontrolled parallel comparative study design. A standard anesthetic technique consisting of thiopental-atracurium for induction and N2O-Enflurane for maintenance of anesthesia was used. The data showed that intravenous ondansetron 4mg i.v. before induction was significantly more effective in preventing emesis (n=20, 85%) than placebo (n=20, 60%). Nausea severity scores were also significantly decreased from 1.5±0.6 (mean±SE) in placebo group to 0.30.1 in ondansetron group (p<0.05). All patients receiving of ondansetron 4mg intravenously tolerated their treatment well, without any special adverse events. Conclusion; Ondansetron(4mg) given intravenously before induction to prevent postoperative nausea and vomiting in patients undergoing elective abdominal surgery was effective and safe.