Post-operative vomiting, especially in ambulatory surgical patients, remains a troublesome problem. This placebo-controlled, randomized, prospective double-blind trial was designed to evaluate the efficacy of two prophylactic antiemetic regimens on post-operative vomiting in 1-day thyroid surgery. Altogether 60 elective surgical patients were followed for 4 h post-operatively. All patients were American Society of Anesthesiologists physical status of I or II and aged between 22 and 60 years: group 1 was saline control; in groups 2 and 3, metoclopramide (0.2 mg/kg) or tropisetron (5 mg) was administered, respectively, as an intravenous single dose during induction. Patients were pre-medicated. A standardized anaesthetic technique consisting of thiopentone-succinylcholine for induction and fentanyl-nitrous oxide-halothane-pancuronium for maintenance of anaesthesia was used. A 'rescue' anti-emetic was provided in case of continued vomiting or at the patient's request. Anti-emetic inefficacy was defined as request for rescue anti-emetic and/or vomiting episode during the first 4 h post-operation. The number of patients vomiting was 12/20 (60%), 10/20 (50%) and 1/20 (5%) within the first 2 h post-operation in groups 1, 2 and 3, respectively (P > 0.05 for groups 1 and 2; P < 0.01 for groups 2 and 3; P < 0.001 for groups 1 and 3). In group 2, three patients required rescue medication during the first 2 h post-operation, but no significant difference was observed between groups 2 and 3 (P > 0.05). None of the cases in any of the groups needed any rescue medication during postoperative 2 - 4 h. It is concluded that tropisetron is a highly effective anti-emetic drug in the prophylaxis of postoperative vomiting.