The use of Doppler sonographic equipment transmitting pulsed high-frequency (20 MHz) ultrasound in conjunction with miniaturised probes, allowed recordings from small brain vessels having diameters of 1 mm and less. Experience made with this technique in clinical and experimental neurosurgery demonstrated its value in all types of neurovascular operations. Nearly 10% of the immediate results of such operations, though apparently successful or without adverse findings, eventually proved unsatisfactory on Doppler sonography only and could be subsequently corrected. The quality of arterial anastomoses performed in animal experiments as well as the various anastomotic techniques can be investigated in terms of their haemodynamic efficiency. The same holds true for cerebral arterial anastomoses in man. The patency of these anastomoses can be examined intraoperatively and necessary corrections can be made immediately. Similarly the method is most helpful in aneurysm operations by allowing to verify the clipping of the aneurysm neck and the patency of the supplying vessels.