In the present study cerebral blood flow was assessed in 40 subjects with chronically low blood pressure and 40 normotensive controls at resting conditions and during the execution of a cued reaction time task. Blood flow velocities were recorded by means of transcranial Doppler sonography in both middle cerebral arteries. In hypotensives flow velocity at rest was reduced bilaterally. During the anticipation of the stimuli, which the subjects had to respond to, a predominantly right hemispheric increase of flow velocity was observed in both groups. This increase was significantly less pronounced in the hypotensive group. Hypotensives showed longer reaction times, and there was a negative correlation between the extent of the flow velocity increase and the reaction times. This study is the first to demonstrate a reduced cerebral perfusion and maladaption of blood flow to cognitive demands due to essential hypotension.