Patient description: The patient is a 20 year old Caucasian female presents to the clinic with one-sided vision lost and facial weakness, dysarthria and numbness lasting for 1 day. She visited her PCP and underwent brain MRI which revealed a single plaque in the brainstem. After few months, she experienced lower extremities weakness led to balance problem. The second MRI revealed another lesion in the left cerebral hemisphere. The diagnosis of Relapsing Remitting Multiple Sclerosis (RRMS) is confirmed after the second MRI. Her past surgical history is significant for a C-section 2 years ago and she has a one child. She is divorced and is not currently in any sexual relationship. She smokes 10 cigarettes per day and drinks alcohol occasionally. She is working as an editor in a publisher company and she is happy that she can keep working from home most of the time. She is under the treatment of RRMS from 7 months ago.

Clinical trial eligibility criteria: eligible ages (years): 0.0-999.0,  Multiple Sclerosis Any healthy normal volunteer above the age of 18 who is capable of giving informed consent recruited or self referred through the NIH Volunteer office will be eligible for this study. All healthy normal volunteers will be included as long as there is no recorded or documented signs or symptoms of CNS disease, contraindications to a MRI and have a "normal age appropriate" MRI of the brain. Patients seen in the Neuroimmunology MS clinic with a confirmed diagnosis of Multiple Sclerosis based upon previous history of two clinical neurological attacks separated in time and in spatial location or combination of Clinical and MRI findings of a single enhancing lesion in the brain or spine along with multiple T2 hyperintensities in the juxtacortical, periventricular or infratentorial white matter according to the McDonald will be included in this study. Relapsing-remitting or secondary progressive MS who have had more than one relapse within 18 months preceding study enrollment will be recruited from the MS 7th floor clinic in the NINDS at the NIH. MS patients will have EDSS score between 1.0 5, inclusive. MS patients seen and treated in the NINDS MS clinic are representative of the general MS population that is Female: male ratio of approximately 3:2, Caucasian and African American, between ages of 18-55 years old. Give written informed consent prior to any testing under this protocol, including screening/pre-treatment tests and evaluations that are not considered part of the patient's routine care  Healthy Controls and MS patients will be excluded if they have contraindications to MR scanning, such as the following: 1. aneurysm clip 2. implanted neural stimulator 3. implanted cardiac pacemaker or autodefibrillator 4. cochlear implant 5. ocular foreign body (e.g., metal shavings) 6. insulin pump Healthy controls and MS patients will be excluded from this study if they have the following: 1. History of heart attack 2. History of treatment with nitrates for heart condition 3. History of carotid artery stenosis or evidence of greater than 50% carotid stenosis on screening MR angiogram 4. History of known vascular disease 5. History of stroke 6. History of migraine 7. Subjects who have a history of a reaction to MR contrast agents specifically gadopentetate dimeglumine will be excluded from participating in the contrast agent administration part of this protocol. 8. Healthy controls will be excluded if have history of alcohol or drug abuse. 9. Healthy controls will be excluded if Concurrent, clinically significant (as determined by the investigator) gastrointestinal, immunologic, pulmonary, neurologic, renal, and/or other major disease. 10. Healthy controls will be excluded if they have a previous known abnormality on Brain MRI examination. 11. Pregnant and lactating women will be excluded from the study. 12. Since certain drugs may interfere with the ability of the vessels in your brain to respond to Viagra, subjects taking steroids or sildenafil (within 24 hours) will be excluded from this study. MS patients will also be excluded from study entry if any of the following exist at the time of enrollment: 1. Pregnant and lactating women who are MS patients will be excluded from the study. 2. Diagnosis of primary progressive MS, defined as gradual progression of disability from the onset without relapses. 3. Concurrent, clinically significant (as determined by the investigator) immunologic, pulmonary, gastrointestinal, neurologic, renal, and/or other major disease in MS patients. Treatment History in MS patients If prior treatment with steroids was received, the subject must have been off treatment for the required period prior to enrollment (see below). Agent: Corticosteroids Time Required off Agent Prior to Enrollment: 8 weeks Unwillingness or inability to comply with the requirements of this protocol including the presence of any condition (physical, mental, or social) that is likely to affect the subject's returning for follow-up visits to the NINDS, Neuroimmunology Clinic will be sufficient reason to a subject