The glucagon stimulation test has been superseded in recent years by the clonidine suppression test because it can provoke dangerous increases in blood pressure in patients with pheochromocytomas. We describe the first patient in whom a pheochromocytoma was diagnosed by a glucagon test, after which the blood pressure (but not the plasma catecholamine) response to a second injection of glucagon was blocked by pretreatment with phenoxybenzamine. After the tumor (which contained both pheochromocytoma and ganglioneuroma tissue) was removed, a third glucagon test result was negative. This experience suggests that patients with normal plasma catecholamine levels who are suspected of harboring a pheochromocytoma may be accurately diagnosed, but potentially dangerous increases in blood pressure may be minimized, by performing the glucagon test after alpha-adrenergic blockade.