The effects of insulin-induced hypoglycemia on catecholamine secretion were investigated in patients with various neurological disorders affecting the autonomic nervous system. In control subjects, insulin-induced hypoglycemia resulted in marked increases in plasma epinephrine and norepinephrine levels. Heart rates were increased within 15 minutes after the insulin injection which were associated with slight elevation and depression of systolic and diastolic blood pressure, respectively. In patients with upper level spinal cord lesions (C1-T6) of various etiology, Shy-Drager syndrome and familial amyloidosis, insulin-induced hypoglycemia failed to increase plasma epinephrine and norepinephrine levels and resulted in falls in systolic and/or diastolic blood pressure 15 minutes after the injection. Heart rates were increased at 30-45 minutes after the injection. In patients with lower spinal cord lesions (T10-L1), neurosyphilis or brain stem tumor with orthostatic hypotension, the catecholamine responses were normal and blood pressure did not fall during insulin-induced hypoglycemia. In patients with Parkinson's disease and spinocerebellar degeneration with autonomic symptoms catecholamine responses were not impaired. These findings suggest that any lesion involving the sympathetic efferent systems of baroreflex such as the spinal descending pathway, sympathetic preganglionic neuron and peripheral nervous system causes both impairment of catecholamine secretion and a fall in blood pressure during hypoglycemia, and that lesions in sympatho-afferent system may not affect the secretion of catecholamine and neural control of blood pressure.