Pheochromocytoma is often associated with paroxysmal hypertension. Wereport a 49-yearold womanwith pheochromocytoma whose blood pressure (BP) was elevated regularly only at night. Plasma norepinephrine (NE) and neuropeptide Y (NPY) concentrations increased in parallel with the elevation ofBP. After resection of the adrenal tumor, these circadian changes disappeared.Plasma NEand NPY, especially the former, from the tumor were considered to be the cause of this unusual fluctuation in BP. (Internal Medicine 32: 781-783, 1993)