Changes in cerebral vascular resistance were examined in patients with cerebral circulatory diseases by the Doppler ultrasonic technique after administration of a cerebral vasodilator, vinpocetine, for two months. Continuous index (CI) and pulsatility index (PI) of the blood flow pattern in the internal carotid artery were used as objective parameters for changes in cerebral vascular resistance. 1. The CI and PI changed significantly after administration of the drug; ie, the CI increased while the PI decreased. 2. An inverse correlation was noted between the rate of change of the CI (delta CI) and that of the PI (delta PI). 3. The results suggest that measurement of the CI and PI by the Doppler ultrasonic technique is useful in investigating the effect of drugs on the cerebral circulation.