Pheochromocytoma is a tumour of the:

Most adrenal medullary tumors (pheochromocytomas) secrete norepinephrine or epinephrine or both and produce sustained hypertension. Pheochromocytoma is a tumour of the adrenal medulla. Reference: Ganong’s Review of Medical Physiology T W E N T Y - F I F T H   E D I T I O N page no 356