Crouching then standing induces large changes in arterial blood pressure. Ingestion of carbohydrate and water are known to have contrasting effects on cardiovascular control in patients with various forms of autonomic dysfunction. We tested the hypothesis that, in patients with attacks of posturally related (neurogenic or vasovagal) syncope, postural maneuvers would cause greater changes in blood pressure than in normal controls and that they would be affected differently by carbohydrate and water. We studied 7 patients with histories indicating posturally related syncope and who we had shown to have abnormal responses to an orthostatic stress test, and 7 healthy volunteers with normal orthostatic responses. Responses of blood pressure (Portapres finger photoplethysmography) were determined to crouching and subsequent standing, before and after ingestion of a high carbohydrate "meal" (2.5MJ) and (on a different day) water (500 ml). Before the "meal" or water there were no differences between groups in baseline blood pressures or in the responses to crouching and standing. In controls, neither carbohydrate nor water had any significant effect on baseline blood pressure or on the responses to the maneuvers. In the patients, the standing pressures were also unaffected by carbohydrate but they did increase after water. In the patients the increases in pressure during the crouch were larger after carbohydrate, but smaller after water. These results show that, in patients with posturally related syncope, unlike in control subjects, carbohydrate ingestion and water result in opposite effects on blood pressure during postural maneuvers. These results raise the possibility that these patients might have a mild form of autonomic dysfunction.