Healthy subjects and patients with multiple sclerosis who are between 18 and 60 years of age may be eligible for this study. Patients must not have severe clinical disability and must have been receiving and responding to Interferon beta for at least 6 months prior to enrollment. Candidates are screened with a medical history, physical examination, MRI and possibly evoked potential testing, which measures the nervous system response to visual, auditory and somatosensory stimulation.