 Physiological outcome measures include hemodynamic responses to the Valsalva maneuver, orthostasis, and altered temperature at skin of the back. Neuropharmacologic measures include cardiovascular responses to test drugs that probe specific components of the autonomic nervous system. Neurochemical measures include plasma, cerebrospinal fluid, microdialysate, urine, and skin biopsy tissue levels of catecholamines and related compounds. Neuroimaging measures include positron emission tomographic scanning after injection of 18F-dopamine, 18F-DOPA, 13N-ammonia, or 11C-methylreboxetine. 