SummaryThis randomized, double blinded, placebo controlled, prospective study compared the antiemetic efficacy of one preoperative dose of metoclopramide 0.25 mg·kg−1intravenously or ondansetron 0.15 mg·kg−1intravenously with two doses of the same drugs (second dose administered one h postoperatively) in 200 preadolescent children undergoing tonsillectomy with either isoflurane or propofol anaesthesia. The incidence of posttonsillectomy vomiting was significantly reduced (P<0.005) by two doses of either metoclopramide or ondansetron (18% and 8%, respectively) compared with placebo (50%). No difference in posttonsillectomy vomiting exists between the children who received isoflurane and those who received a propofol infusion. Our results suggest that two doses of metoclopramide 0.25 mg·kg−1intravenously, like two doses of ondansetron 0.15 mg·kg−1, are effective in reducing vomiting after tonsillectomy in children who have received either isoflurane or propofol