Orthostatic Intolerance is a syndrome more common in women than men and typically seen in younger rather than older patients. It is typified by postural tachycardia. Orthostatic symptoms include lightheadedness, fatigue, nausea, tremulousness, and alterations in vision such as tunnel vision or blurring. Less commonly patients complain of anxiety, headache, acral pain or coolness, and chest wall pain. Patients sometimes have postural hypotension although this isn't as marked as the postural tachycardia would suggest. Up to fifty percent of patients have had an antecedent viral illness prior to developing orthostatic intolerance. This willl assess renal hemodynamics and other indicators of sympathetic activity in response to head up tilit and Trimethaphan infusion in orthostatic patients with low plasma renin activity, patients with high plasma renin activity and normals, and assess response to a low sodium diet in orthostatic intolerance patients with low plasma renin activity, patients with high plasma renin activity and normals.