Objective - To determine whether a continuous time estimation task during test occlusions of either internal carotid artery would increase the ability to detect the earliest signs of cerebral ischaemia in the anterior circulation. Methods - Four patients were involved in real time measurement of their timing accuracy before, during, and after each test occlusion. While under each test condition, patients were instructed to press a mouse button connected to a computer and then to press it again no sooner than 10 seconds from the previous response but no longer than 13 seconds later. While being given automated feedback on accuracy, patients were instructed to continually press the mouse on the target schedule to maximise correct responses until told to stop. Results - The data showed deterioration of timing accuracy during carotid occlusion (P < 0.05), which always preceded the onset of physical signs and correlated in one patient with the presence of reduced regional cerebral blood flow. Conclusion - Decline of sustained attention under conditions of test balloon occlusion of either internal carotid artery was an indicator of failure to maintain adequate cerebral blood flow to sustain normal neurological function. The demonstration of the behavioural effects of early cerebral ischaemia shows the feasibility of an experimental model for the study of human brain function, and may now make it possible to quantify more precisely the time course of acute ischaemic events.