IOH occurs as progressive autonomic failure (PAF) without any neurologic symptoms indicating multiple system atrophy or Parkinson's disease. The responsible lesion for IOH is yet obscure but has been suggested to be in the peripheral sympathetic nerves, since postganglionic sympathetic neurons in IOH fail to release norepinephrine and there present extensive supersensitivities to exogenous pressors. SOH is characterized as marked tachycardia induced by hypotensive stress like standing, and is less sensitive to the administered catecholamines. Careful examinations by some pharmacological studies are essential to diagnose IOH and SOH in patients with orthostatic hypotension.