Hemodynamic features of the posterior circulation were evaluated by assessing visual evoked response in posterior cerebral artery (PCA) by means of functional transcranial Doppler (fTCD) in 49 patients with high-grade (70%-99%) internal carotid artery (ICA) stenosis or occlusion and 30 healthy subjects. Mean blood flow velocities (MBFV) and mean reaction time (time to peak velocities) (MRT) in each PCA were measured in the dark (closed eyes) and during white light stimulation (opened eyes, looking at the electric bulb), during three consecutive repetitive periods of 1 minute each. In the group of severe carotid disease patients, there was no difference in MRT in PCA during the white light stimulation (P=0.1), whereas in the dark MRT values showed a statistically significantly prolonged visual evoked response (P=0.02), but with no clinical relevance. MBFV values did not differ significantly during white light stimulation (P=0.1), whereas in the dark the difference was also statistically significant (P=0.03), but with no clinical relevance. On the contrary, in the group of healthy subjects, MRT values differed significantly both during white light stimulation (P=0.0005) and in the dark (P=0.00054), showing a significantly prolonged visual evoked response. During white light stimulation, MBFV showed significant decrease and prolonged vasoreactive response (P=0.004). Prolonged vasoreactive response in PCA in healthy subjects during repetitive measurements may indicate exhaustion of the vasoreactive mechanisms. In carotid disease patients, stable vasoreactive response may indicate that the compensatory mechanisms of the posterior circulation are always maximally engaged to compensate for carotid insufficiency.