The purpose of this investigation was to assess whether the phentolamine test is able to predict blood pressure fluctuations during operations on patients with pheochromocytoma. We administered 5 mg phentolamine intravenously to 11 patients with pheochromocytoma and 6 patients with essential hypertension, pretreated with alpha‐adrenoceptor antagonists. Plasma concentrations of epinephrine and norepinephrine were determined before and after phentolamine administration, and blood pressure was measured repeatedly. Subsequently, all patients with pheochromocytoma underwent adrenal operations. Evidence is presented to show that: (1) in patients with pheochromocytoma whose blood pressure remains stable during their operation, phentolamine produces no clear effects on mean blood pressure (MBP), plasma epinephrine and plasma norepinephrine except in one patient with Sipple's syndrome, (2) in patients with pheochromocytoma exhibiting unstable blood pressure changes during surgery, phentolamine lowers MBP and raises plasma norepinephrine and (3) in patients with essential hypertension, MBP and plasma catecholamines remain unchanged following phentolamine administration. Based on these findings, this pharmacological test appears to be useful for predicting hemodynamic stability in patients with pheochromocytoma pretreated with alpha‐adrenoceptor antagoni