This pilot study is optional and will be conducted in patients already enrolled in the "Evaluation and Treatment of Autonomic Failure" and the medication trial part of this protocol. Subjects will be studied in the supine position on two study days (with and without heat stress). Each study day will last ~3 hours. Core body and skin temperature will be monitored throughout the study using an ingestible telemetry pill and dermal patches. Blood pressure and heart rate will be measured intermittently with an automated blood pressure sphygmomanometer wrapped around an upper arm. Segmental body fluid shifts will be estimated using bioelectrical Impedance and hemodynamic parameters using body impedance and the rebreathing test (Innocor). After obtaining normothermic baseline measurements, we apply passive heat-stress with a commercial heating pad that covers all the abdomen and part of the torso to provide local heating at ~44ºC continuously for 2 hr. Outcome measurements are obtained at 1 and 2 hours after passive heat-stress, or when the CBT increases ~1ºC above baseline, whichever occurs first. For the control (non-heating) study day, the heating pad will be applied on patients but we will not turn it on, and data collection will be performed at the one-hour intervals for 2 hours, to provide a time control.