Insulin-induced hypoglycemia induced a fall in blood pressure (BP) in patients with idiopathic orthostatic hypotension (IOH) and multiple system atrophy (MSA), but not in control subjects. Only in IOH was there a correlation between plasma norepinephrine (NE) levels and maintenance of BP during the test. The hypotension was not affected by pretreatment with propranolol. Hypotension during insulin-induced hypoglycemia is manifested in patients who lack an adequate NE response. The hypotension, however, may be due to a central action of insulin because not all MSA patients with impaired NE release become hypotensive.