In 100 normal subjects mean flow velocity in the middle (MCA), anterior (ACA), and posterior (PCA) cerebral arteries and mean pulsatility index were examined by transcranial Doppler ultrasound. Age-dependent normal reference values were assessed. The measured parameters showed a large variation increasing with age. The highest flow velocities were found in the MCA, the lowest in the PCA. The highest mean side difference in flow velocity was observed in the ACA. An increase of pulsatility index and a decrease of flow velocities in all examined vessels were found with increasing age. The influence of ventilation and heart function on flow velocity and pulsatility index was demonstrated by the investigation of 10 normal subjects in different phases of ventilation and 1 patient with aortic valve insufficiency.