Hemodynamic responses to reversal of phenoperidine-nitrous oxide anesthesia were studied in 14 adult patients before and after naloxone administration (1.5 +/- 0.25 microgram/kg), and, at comparable intervals, in 11 control patients who were permitted to resume respiration spontaneously. Naloxone reversal resulted in significant increases in heart rate (31%), cardiac index (50%), left ventricular stroke work index (53%), and systemic blood pressure (21%), compared with initial values. The heart rate-systolic arterial pressure product, an indirect index of myocardial oxygen consumption, increased significantly (77%). However, changes of similar magnitude occurred after spontaneous recovery in control patients, in whom the only significant treatment-related difference was a longer recovery time. Whether naloxone is used or not, the observed hemodynamic changes may be harmful to patients who have diminished cardiac reserve.