Plasma concentrations of norepinephrine (NE) appear to reflect sympathetic nervous system (SNS) tone. Plasma NE increases substantially when the SNS is stressed by acute postural change or physical exercise. Standing up from the recumbent position elevates NE by about 90% in healthy normotensive and hypertensive subjects. Although a primary role of the SNS is the maintenance and stabilization of blood pressure, there does not seem to be a direct relationship between plasma levels of NE and blood pressure. Patients with essential hypertension cannot be differentiated from normotensive subjects of the same mean age on the basis of either basal plasma levels of NE nor increments in NE after a standard postural stress. Patients with syndromes affecting the autonomic nervous system who suffer from orthostatic hypotension may have striking abnormalities in SNS response to stress, and some subgroups may be distinguished on the basis of the plasma concentrations of NE and the changes in NE elicited by stress.