Transcranial Doppler monitoring of the flow velocity at the level of the middle cerebral artery was performed in 40 demented patients, 20 with multi-infarct dementia (MID) and 20 with senile dementia of the Alzheimer type (SDAT), and in 25 age-matched controls. The following conditions were evaluated: (1) rest; (2) 60 s hyperventilation; (3) longest possible apnea, and (4) 5 min closed-circuit air rebreathing. We also measured: PaCO2 levels at rest and under stimulus conditions; mean flow velocity and pulsatility index (PI) at rest, and percentage velocity variations. The PIs were higher and the velocity decrease during hyperventilation was lower in all demented patients than in the healthy group; no side-related asymmetry in rest values or in vasomotor responses to CO2 changes was regularly detected in any group. On the contrary, rest flow velocities and vasomotor responses to hypercapnia induced by both apnea and rebreathing tests proved to be lower in MID patients than in SDAT and healthy groups. These alterations were neither exclusive to MID patients nor homogeneous, therefore some caution should be taken when evaluating single cases.