Ultrasonic duplex scanning was used for flow velocity measurement in the extracranial carotid vessels in 92 presumably healthy volunteers aged 20-82 and in 14 patients aged 23-72 with normal carotid vessels on angiography. Velocities were examined as a function of age, diameter, blood pressure, side, or position in the vessel. The peak systolic velocities and late diastolic velocities in the CCA showed an age-dependent decrease (p less than 0.001), and the same was true of the peak systolic velocity in the proximal ICA on the right side (p less than 0.05) and the late diastolic velocity in the right (p less than 0.01) and left (p less than 0.001) proximal ICA. The change in velocity was inversely correlated to diameter and blood pressure. The systolic and late diastolic velocities were significantly (p less than 0.05 and 0.001 respectively) faster in the left CCA at low position than in the right, and, conversely, in the right ICA (p less than 0.05) than in the left. At CCA high position faster diastolic velocities were noted than at CCA low, on both sides. All age-dependent quotients comprised a peak systolic velocity in the CCA, whereas the quotient between the late distolic velocities in ICA and high did not show age dependency. Reproducibility of peak systolic and late diastolic velocities in the CCA, ICA, and ECA, expressed as the mean percentage difference, varied between 6.4 +/- 4.6% and 12.7 +/- 6.4%.