Postoperative nausea and vomiting is a major concern for patients undergoing outpatient surgery under general anaesthesia, and may complicate and delay discharge from hospital. This paper evaluates the safety and efficacy of ondansetron, a 5-HT3 receptor antagonist, in the treatment of postoperative nausea and vomiting. One thousand patients in 30 centres in the United States who received general anaesthesia and developed postoperative nausea and vomiting were studied. In a randomised, double-blind, stratified and parallel designed protocol, patients received either ondansetron 1, 4, 8 mg or placebo for nausea or vomiting occurring within 2 h of entry into the Post Anaesthesia Care Unit. Subsequent episodes of vomiting, nausea scores, laboratory and clinical safety data and adverse events were evaluated during the 24-h study period. In a separate study, pharmacokinetic data were compared for intramuscularly and intravenously administered ondansetron. Each dose of ondansetron was significantly better than placebo in reducing nausea from control values during the initial 2-h study period, and in preventing further emesis over 24 h. There were no significant differences in the incidence of adverse events, changes in laboratory values or measures of vital signs in the ondansetron groups compared to the placebo group. Dose comparisons between the three treatment groups showed that ondansetron 4 mg is the optimal dose to treat postoperative nausea and vomiting. Ondansetron is a well tolerated, efficacious antiemetic which has a similar side effect profile to placebo. Intramuscular administration has the same systemic availability as intravenous administration.(ABSTRACT TRUNCATED AT 250 WORDS)