In eight patients undergoing carotid endarterectomy, the mean velocity and an index of pulse amplitude in the middle cerebral artery were monitored continuously by transcranial doppler ultrasound. rCBF was measured by intracarotid injection of 133 Xenon shortly before and at the time of the carotid artery occlusion, and again a few minutes after carotid flow was reestablished. Comparison of the mean velocity in the MCA and the cortical convexity rCBF revealed relatively little hysteresis in their relationship from prior to after the occlusion. There was however, considerable variability in this relationship among patients. Both the rCBF and the velocity decreased substantially at occlusion in three cases, neither changed very much in three. While in two, though the rCBF decreased significantly, the velocity did not change. The index of pulse amplitude was somewhat more sensitive to the occlusion, decreasing in the seven cases in which it was recorded, including one in which the rCBF did not change.