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Hypotension was induced in sixteen patients with nitroprusside during anesthesia for surgical correction of cerebral aneurysms. Eight patients were premedicated with propranolol (180 mg orally) for one day and the other eight were not. Before the start of anesthesia, the untreated patients had a mean arterial pressure of 102 ± 3 mm Hg, a heart rate of 76 ± 2 beats/min, and plasma epinephrine and norepinephrine concentrations of 114 ± 21 and 258 ± 34 pg/ml, respectively. The propranolol-premedicated patients came to the operating room in a significantly different clinical state with a mean arterial pressure of 92 ± 3 mm Hg, a heart rate of 71 ± 2 beats/min, and plasma epinephrine and norepinephrine concentrations of 76 ± 28 and 144 ± 28 pg/ml. During induced hypotension, plasma epinephrine and norepinephrine concentrations increased significiantly in both groups (454 ± 42 and 730 ± 58 pg/ml in the untreated patients, 160 ± 48 and 419 ± 67 pg/ml in the propranolol premeditated patients), but the increase in catecholamines was significantly greater in the untreated patients. Thirty minutes after nitroprusside was discontinued, epinephrine and norepinephrine concentrations were higher than in the awake state in untreated patients and were associated with rebound hypertension and tachycardia. In contrast, in propranolol-premedicated patients, plasma epinephrine and norepinephrine concentrations decreased towards the preanesthesia values, mean arterial pressure gradually returned to the prehypotension level, and heart rate remained unchanged. This study thus demonstrates that propranolol premedication attenuates the release of catecholamines in response to nitroprusside-induced hypotension.