Clinical recognition of pheochromocytoma as a cause of hypertension is still a difficult problem for which the many tests that have been devised are at times inadequate. In this article an antidiuretic response to piperoxan in patients with pheochromocytoma is described. In view of the diuretic response of patients without pheochromocytoma, measurement of urinary flow after injection of piperoxan is suggested as a possible simple, confirmatory guide in diagnosis.