Ondansetron, a selective 5-HT3 antagonist has been proved to be an effective antiemetic agent for prophylaxis of nausea and vomiting after surgery. This study was conducted to determine whether ondansetron changes thiopental requirements for induction of anesthesia in patients unpremedicated and premedicated with diazepam. One hundred sixty eight adult female patients classified as American Society of Anesthesiologists (ASA) physical status I (normal healthy patient) or II (patient with mild systemic disease) participated in this prospective, double blinded, randomized study. Patients were assigned to receive either 0.07 mg/Kg diazepam orally or no premedication. They then received saline and ondansetron 0.1 mg/Kg or 0.2 mg/Kg intravenously 5 minutes before thiopental induction. Thiopental was administered at a rate of 25 mg/min until the patient lost the ability to open eyes on command. Thiopental requirements were not significantly different among groups. The results indicate that ondansetron in clinically used doses does not influence the hypnotic requirements of thiopental.