We investigated the effect of a balanced liquid meal on blood pressure (BP) and heart rate (with patients supine and during head-up tilt), and on levels of plasma catecholamines, glucose, and insulin, in patients with idiopathic Parkinson's disease (IPD), multiple system atrophy (MSA), pure autonomic failure (PAF), and in healthy subjects (controls). After food, supine BP fell in IPD, but to a greater extent in MSA and PAF. In controls, BP was unchanged. Head-up tilt did not lower BP in IPD and controls, but there was a postprandial fall to lower levels in both MSA and PAF. Plasma norepinephrine levels rose in IPD pre- and postprandially during tilt, but were unchanged in MSA and PAF. These data suggest that in IPD, food causes a smaller fall in supine BP than in MSA and PAF. In IPD, as in controls, food does not induce or unmask postural hypotension, unlike in MSA and PAF, in which BP falls to even lower levels. There are therefore differences in the responses to food ingestion between these groups. This may be of value in separation of these disorders at an early stage.