The hemodynamic changes of naloxone and nalbuphine following intravenous administra tion of high-dose fentanyl were studied using dogs anesthetized with halothane and ventilated artificially. After 50ug/kg of fentanyl was given to all experimental animals, they were randomly divided into two groups, such as group 1 and group 2. Group 1 and 2 were given 20ug/kg of naloxone and 0.3mg/kg of nalbuphine known as antagonists of fentanyl-induced respiratory depression respectively. Hemodynamic parameters were recorded before, 5 and 30 minutes after fentanyl and 1,10 and 20 minutes after naloxone or nalbuphine. During halothane anesthesia, fentanyl significantly decreased mean arterial pressure, heart rate, cardiac index, rate pressure product, left ventricular stroke work index and right ventricular stroke work index in all dogs(p<0.05). After fentanyl reversal by antagonists, dogs in group 1 promptly developed significant increases above baseline values in mean arterial pressure, heart rate, mean pulmonary arterial pressure, central venous pressure, pulmonary capillary wedge pressure, rate pressure product and left ventricular stroke work index(p<0.05), whereas dogs in group 2 did not show significant hemodynamic changes. These results suggest that the abrupt, untoward and significant hyperdynamic events which accompany narcotic reversal with naloxone can be avoided if nalbuphine instead of naloxone is administered.