Prazosin was used to control the signs and symptoms of excessive alpha-adrenergic activity in four patients with pheochromocytomas. Long-term dosage requirements were predicted by the blood pressure response to a single 1-mg oral dose. However, surgical management of the pheochromocytomas was inadequate with prazosin alone, and intravenous phentolamine was required to suppress the pressor surges generated by the tumor during surgical manipulation and excision.