We have compared the efficacy of ondansetron with droperidol and saline in the prevention of postoperative nausea and vomiting (PONV) in 120 ASA I and II patients undergoing hip and knee replacements and femoral resections. They received a standardized combined extradural and general anaesthetic and at the end of surgery were allocated randomly to receive droperidol 1.25 mg, ondansetron 4 mg or 0.9% saline in a 25-ml bag. An extradural mixture containing 0.5% plain bupivacaine 10 ml, fentanyl 500 mu g and saline 30 ml was infused and PONV assessed for 24 h. Both ondansetron and droperidol were superior to saline in preventing vomiting (P < 0.01) although there was no significant difference between them. The incidence of vomiting was 17% for ondansetron, 18% for droperidol and 45% for saline. There was no significant difference in the incidence of nausea between the groups. Metoclopramide, the rescue antiemetric, was demanded by 38%, 34% and 17% of patients receiving saline, droperidol and ondansetron, respectively (ondansetron vs droperidol P < 0.05).