The chromaffin cells of the adrenal medulla are innervated neuroendocrine cells that secrete catecholaminergic neurohormones into the blood. The biosynthetic steps leading to norepinephrine occur in all the chromaffin cells of the adrenal medulla. In mammals, the phenylethanolamine N-methyl transferase (PNMT) enzyme is almost exclusively localized in the adrenal medulla with only small quantities in the brain and in the sympathetic nerves of the heart. The subset of chromaffin cells that contain PNMT and synthesize epinephrine are regionally localized in a species-specific manner. The primary secretory products of adrenal medullary cells are the catecholamines, norepinephrine and epinephrine. In the discussion on biological actions of catecholamines, the induced breakdown of glycogen to yield glucose was one important action that could only be accomplished by adrenomedullary secretions. Therefore, low blood glucose represents one of the most important provocative stimuli of such secretion.