1. Systemic and regional haemodynamic responses, including superior mesenteric artery blood flow, were measured during stimuli which increase sympatho-neural activity in age-matched normal subjects (controls) and in two groups of patients with sympathetic failure (pure autonomic failure and multiple system atrophy). The stimuli included the pressor tests (mental arithmetic, cutaneous cold and isometric exercise) and head-up tilt. 2. In the controls, the blood pressure did not rise in some during mental arithmetic, but rose in all subjects during cutaneous cold and isometric exercise and was maintained during head-up tilt. In sympathetic failure patients, blood pressure was unchanged during each pressor test and fell during head-up tilt. 3. In the controls, superior mesenteric artery blood flow did not fall significantly during mental arithmetic but fell (with a corresponding rise in calculated superior mesenteric artery vascular resistance), during cutaneous cold, isometric exercise and head-up tilt. In sympathetic failure patients, there were no changes in superior mesenteric artery blood flow and vascular resistance during the pressor tests and head-up tilt. 4. There were no changes in cardiac index or forearm blood flow during each pressor test in both controls and patients. Cardiac index fell and forearm vascular resistance rose during head-up tilt in the controls only. 5. In conclusion, active constriction of the superior mesenteric artery occurs in normal subjects during sympatho-neural activation induced by stimuli such as cutaneous cold, isometric exercise and head-up tilt. This does not occur in patients with sympathetic failure and probably contributes to postural hypotension, emphasizing the role of the splanchnic vascular bed in overall blood pressure control. This study confirms the necessity of integrity of sympathetic pathways in the neural control of the splanchnic vascular bed.