The purpose of this study is to determine whether midodrine works against the symptoms of orthostatic hypotension caused by being on a tilt table.

The efficacy of midodrine will be assessed in those subjects who have severe symptoms of orthostatic hypotension when not taking midodrine and are controlled when taking midodrine. The study will involve approximately 4 overnight stays.

Inclusion Criteria:

1. Male and female subjects must be 18 years of age or older and ambulatory.
2. Females of child-bearing potential (FOCP) must have a negative serum beta human chorionic gonadotropin (HCG) pregnancy test.
3. A documented history of severe Symptomatic Orthostatic Hypotension (SOH) that, in the judgment of the treating physician, has required treatment with midodrine HCl , and has been at a stable dose for at least 3 months.
4. The subject has manifested at least 1 of the following symptoms while standing or had a medical history of 1 of the following when not treated for orthostatic hypotension (OH): dizziness, lightheadedness, feeling faint, or feeling like they might black out.

Exclusion Criteria:

1. The subject is a pregnant or lactating female.
2. The subject has pre-existing sustained supine hypertension greater than 180mmHg systolic and 110mmHg diastolic BP or had these measurements at the Screening Visit. Sustained is defined as persistently greater at 2 separate measurements at least 5 minutes apart with the subject supine and at rest for the 5 minutes.
3. Subjects taking concomitant medications of interest are excluded unless those medications are reviewed and discussed with the Medical Monitor or Study Physician and documented prior to enrolling the subject. If agreement is reached between the Investigator and Sponsor for the subject to continue in the study, all allowed medications should be maintained at a constant dose throughout the study.
4. The Principal Investigator deems any clinical laboratory test (at the Screening Visit) abnormality to be clinically significant
5. The subject has participated in other studies of investigational drugs or devices within 30 days prior to enrollment in this study (other than Study SPD426-406).
6. Current or relevant history of physical or psychiatric illness, any medical disorder that may require treatment or make the subject unlikely to fully comply with the requirements of the study or complete the study, or any condition that presents undue risk from the investigational product or study procedures.
7. The subject has a concurrent chronic or acute illness, disability, or other condition (including significant unexpected laboratory or electrocardiogram [ECG] findings) that might confound the results of the tests and/or measurements administered in this study, or that might have increased the risk to the subject.
8. Known or suspected intolerance or hypersensitivity to the investigational product(s), closely-related compounds, or any of the stated ingredients.
9. Prior enrollment failure or randomization in this study.
10. History of alcohol abuse or other substance abuse within the last year.