23 patients presenting with symptoms of pheochromocytoma were subjected to the clonidine suppression test. In 6 patients, in whom a pheochromocytoma was surgically approved later on, norepinephrine was not suppressed by clonidine. 17 patients in whom a secondary form of hypertension could be excluded by other diagnostic measures showed a marked decrease of plasma norepinephrine. Apart from drowsiness, no adverse side effects were observed. In our hands, the clonidine test proved as a safe and specific test in the diagnosis of pheochromocytoma.