In this study, Ondansetron (tablet) in doses of 4, 8, and 16 mg was given to three groups preoperatively and their efficacy in preventing postoperative emesis were investigated and compared. 80 adult patients aged 17-65 (mean 42.5 ± 2.6) years were divided into 4 groups. Ondansetron groups received 4, 8 and 16 mg oral ondansetron for premedication. Control group didn't receive premedication. Thiopentone and vecuronium were given for induction and O2/N2O/isoflurane for maintenance of anaesthesia in all patients. Emesis was evaluated at 0.-10. min, 10.-60. min, 1.-6. h, 6.-12. h and 12.-24. h during postoperative 24 h period and 4 groups were compared. In the control group incidences of emesis were found 20 %, 35 %, 65 %, 25 %, and 40 % at 0.-10. min, 10.-60. min, 1.-6. h, 6.-12. h, and 12.-24. h respectively. Ondansetron groups had significantly lower incidences of emesis than the control group at all times. In the 4, 8, and 16 mg ondansetron groups there were differences in the incidences of emesis but maximum decreases were found at the 1.-6. h. At that period, emesis incidences were 5 %, 30 %, and 15 % in the 4, 8, and 16 mg ondansetron groups respectively and these values were significantly lower than the control group (p < 0.001, p < 0.05, p < 0.01). When different ondansetron groups were compared with each other, there was a significant difference only between 4 and 8 mg ondansetron groups (p < 0.05). As a result, administration of 4, 8 and 16 mg oral ondansetron preoperatively decreases postoperative emesis and the dose of 4 mg may be preferable to 8 mg and 16 mg because of its similar antiemetic effects, smaller dosage and lower cost.