Morphine and narcotic agonist-antagonists have been used to assist ICU patients in adapting to mechanical ventilation. In this study, 10 mg of morphine and the equipotent doses of synthetic analgesics, 2 mg of butorphanol, 0.6 mg of buprenorphine or 30 mg of pentazocine were administered intravenously to 29 patients requiring a ventilator. Hemodynamic effects on the heart rate, mean arterial pressure (MAP), cardiac index, stroke index, left ventricular stroke work index, mean pulmonary arterial pressure (MPAP), pulmonary capillary wedge pressure (PCWP) and systemic and pulmonary vascular resistance (PVR) were measured. The hemodynamic effects of the four drugs were mild and not statistically significant except for the reduction in PCWP and the increase in PVR after morphine and the increase in MAP and MPAP after pentazocine administration. These doses of the four drugs could be given safely even in critically ill patients. The hemodynamic effects of these analgesics showed a similarity between the administration of butorphanol and morphine, and between buprenorphine and pentazocine. This study demonstrates that morphine and butorphanol are preferred to the cases with hypertension, increased pulmonary arterial pressure or wedge pressure and that pentazocine and buprenorphine are more suitable for the cases with hypotension or hypovolemia.