The potency of Ondansetron (Zofran, Glaxo), a highly specific 5HT3 antagonist in preventing the very unfavorable complication during introducing anesthesia, i.e. a Bezold-Jarisch reflex-like reaction, was studied in a clinical trial. A total of 20 patients (12 males and 8 females aged 19-65 years) admitted for clinical surgical treatment participated in the trial. Zofran (8 mg in 500 ml Ringer solution) was injected intravenously one hour before intubation at a flow rate of 8 ml. min-1. Systolic and diastolic blood pressure, central venous pressure, breathing frequency and minute ventilation, pCO2 and pO2 in venous blood, electrocardiogram monitoring (ECG) and several common features such as spontaneous muscle activity, palpebral reflexes, skin and mucose surfaces were continuously observed. Visual, auscultatory and X-ray control of the lungs was effected. It was found that blockade of 5HT3 receptors prevented the appearance of Bezold-Jarisch reflex-like reaction. The drug could successfully be used in anesthesiological practice.