The respiratory and hemodynamic responses to postoperative reversal by naloxone of high-dose fentanyl anesthesia were studied in 101 patients after open heart surgery. Respiratory and hemodynamic changes after drip infusion of naloxone were minimum. Change of systolic blood pressure (BPs), mean blood pressure (BPm), CVP and PO2 were statistically significant. BPs increased for 3.8 +/- 14.0 mmHg, BPm increased for 1.6 +/- 8.2 mmHg. CVP decreased for 0.4 +/- 2.1 mmHg, and PO2 decreased for 8.6 +/- 3.4 mmHg. Severe side effect was not observed. Multi-factorial analysis revealed that the abrupt recovery of consciousness from anesthesia and acidosis were the most important factors that attenuate hemodynamic response to naloxone.