The hemodynamic and catecholamine response to intravenously administered naloxone, 0.2 mg and 0.4 mg, were determined in one group of surgical patients and one group of volunteers. Naloxone, 0.2 mg, was administered 30 minutes before the 0.4-mg dose. Group I consisted of six normotensive (IA) (aged 18 to 64, mean 35.7 years) and six hypertensive surgical patients (IB) (aged 35 to 67, mean 49.1 years) who were receiving a nitrous oxide, oxygen, and halothane anesthetic. Group II consisted of six normotensive (IIA) (aged 27 to 38, mean 36.1 years) and five hypertensive (IIB) unanesthetized volunteers (aged 41 to 60, mean 51.4 years). Mean arterial pressure, heart rate, plasma norepinephrine, epinephrine, and dopamine levels were compared before and after intravenous naloxone. Changes in mean arterial pressure, heart rate, plasma norepinephrine, epinephrine, and dopamine levels were not statistically significant in any group. This study suggests that intravenous naloxone, per se, alters neither mean arterial pressure, heart rate, or plasma catecholamine levels in normotensive or hypertensive humans.