Transcranial pulsed Doppler analysis of blood velocity in the middle cerebral artery was performed in 120 healthy volunteers (age 20-70 y, 12 male and 12 female subjects per decade), meeting strict selection criteria. The intention was to create normative reference data, both at rest and during hyperventilation, for the assessment of abnormality in patients with cerebral vascular disorders. The measured blood velocity at rest decreased significantly with increasing age. Females up to 50 years of age had significantly higher blood velocity values than males. Blood velocity diminution was induced by means of voluntary hyperventilation, under capnographic control. An age related decline of blood velocity as present at rest was not found during hyperventilation, thus the relative value of hyperventilation induced changes diminished with increasing age. The pCO2 related change in the blood velocity index appeared not to be a constant value, as suggested by previous authors. The blood velocity index was largest in the change from resting condition to 4 kPa pCO2, and smallest in the change 3 kPa pCO2 to 2 kPa pCO2. When examining the blood velocity in the MCA, the age, sex and end-tidal pCO2 pressure have to be taken into account for a correct interpretation of the data obtained.