A computer-controlled 14-channel pulsed Doppler ultrasound instrument was used as a noninvasive means to evaluate instantaneous velocity profiles and flow in the common carotid arteries of 22 healthy persons and in 22 patients. Of the latter, 13 had severe obstructions of the extracranial portion of the carotid artery, four had obstruction of the intracranial portion, and five had severe aortic valve insufficiency (AI), with more than 60% regurgitation in all cases. Measurements could be performed within an accuracy of about ± 20% under clinical conditions and revealed perfusion values of 5 to 8 ml per second (300 to 480 ml per minute) in healthy persons. Of the patients, values less than 3 ml per second (180 ml per minute) were detected in nine, between 3 and 5 ml per second (180 to 300 ml per minute) in four, and normal values in another four, while significant backflow was observed only in patients with severe AI. Consecutive blood flow profiles were recorded every 4 msec; these demonstrated that, for a period lasting from 40 msec to 280 msec after the initial systolic peak, blood flowdecelerated more rapidly in the central portion of the vessel lumen than near the wall. This situation was present in all healthy persons and in most patients with pathological flow. In patients with AI, flow reinversion from reverse to normal began near the vessel wall, while in the middle third of the vessel lumen, blood was still flowing backward. These phenomena seem to be in agreement with the theoretical and experimental findings of Wormersly, MQIIer, and others. The AI patients who underwent artificial valve implantation were studied ten days after operation and showed no or little backflow in the common carotid artery at that time.