Given the hypothesis that interaction of succinylcholine with nicotinic receptors releases endogenous catecholamines, plasma levels of epinephrine and norepinephrine were determined in anesthetized and manually ventilated patients immediately before and 2 mm after intravenous administration of succinylcholine. Anesthesia was induced with intravenous thiopental (3–4 mg/kg) followed by the administration of nitrous oxide and oxygen (1:1) and 0.5–1.0% halothane. Stimulation of the patients was avoided. Succinylcholine (1 mglkg) or metocurine (0.3 mg/kg) was injected intravenously and ventilation was controlled without intubation. Plasma norepinephrine levels increased from 301 pg/ml to 491 pg/ml (SEM = ±19 pg/ml, P < 0.01, N = 5) 2 min after the injection of succinylcholine; the increase in plasma epinephrine was not statistically significant. The time course of catecholamine elevation was studied in three additional patients. The increase of norepinephrine occurred immediately after the injection of succinylcholine, peaked (647 ± 67 pg/ml) around the third minute, and disappeared by the 10th min. The increase in epinephrine was less marked. Plasma levels of catecholamines did not change after the injection of metocurine (N = 2). The possibility that succinylcholine stimulates nicotinic receptors on the postganglionic sympathetic terminals is discussed. We propose that the elevation of plasma norepinephrine might contribute to the development of early adverse cardiovascular reactions to succinylcholine.