All patients will have neurogenic OH or POTS, defined using the following criteria:

Inclusion Criteria for OH:

• Males or females age 18-75 years.
• Orthostatic BP drop 30 mmHg systolic or 15 mmHg diastolic within 3 minutes of HUT.
• Orthostatic symptoms including weakness, lightheadedness, blurred vision, and difficulty with concentration and thinking, rated at least at 2 in severity on a numeric scale from 0 to 10.

All three criteria must be fulfilled.

Inclusion Criteria for POTS:

• Males or females age 18-55 years.
• Orthostatic heart rate increment 30 bpm within 5 minutes of HUT without the presence of OH.
• Symptoms of orthostatic intolerance. These include weakness, lightheadedness, blurred vision, nausea, palpitations and difficulty with concentration and thinking. These symptoms must be rated at least at 2 in severity on a numeric scale from 0 to 10.

All three criteria must be fulfilled.

Exclusion Criteria for OH and POTS:

• Pregnant or lactating females – a pregnancy test will be required for women of childbearing potential.
• The presence of failure of other organ systems or systemic illness that can affect autonomic function or the patient's ability to cooperate. These include dementia, pheochromocytoma, heart failure, hypertension, renal or hepatic disease, severe anemia, alcoholism, malignant neoplasms, hypothyroidism, sympathectomy, or cerebrovascular accidents.
• Concomitant therapy with anticholinergic, alpha- and beta-adrenergic antagonists or other medication which could interfere with testing of autonomic function.
• Clinically significant coronary artery disease.
• Conditions that increase the potential risk of seizures or cardiac arrhythmias (epilepsy, prolonged QT syndrome) – subjects enrolled in the study will undergo EKG and EEG, and will be excluded if there is evidence of prolonged QT syndrome or of potentially epileptiform discharges, or if there is a history of prolonged QT syndrome, cardiac arrhythmias, or epileptic seizures.