The incidence of multi-symptom illness or Gulf War Illness (GWI) in the Veterans deployed during the Gulf War is estimated to be 25-32 percent. The primary goal of this project is to examine cerebral blood flow responses to chemical and metabolic stress in Veterans with GWI compared with age and deployment-matched Veterans. We hypothesize that GWI is associated with both vascular dysfunction in the cerebral circulation and autonomic dysfunction. Participants will take part in a laboratory visit for autonomic function testing, and a magnetic resonance imaging scan (MRI) visit to determine brain structure and intracranial blood flow measurements at rest and in response to physiological stress. These experiments represent a novel and comprehensive approach and address fundamental and significant unresolved physiological questions in how GWI affects the human brain, with relevance to GWI symptoms.