Patients suspected of having a pheochromocytoma will be studied via a series of tests in an attempt to ascertain biochemically whether or not they really have such a tumor. These procedures will include a standard clonidine suppression test and a standard glucagon stimulation test. Once the diagnosis has been made on the basis of biochemistry, then localization and therapy will be done via standard procedures. Measurement of plasma metanephrines on mailed samples is available for physicians who seek further evidence for the diagnosis of pheochromocytoma.