The relationship between maximal contrast density in DSA and blood flow in doppler sonography was examined in tube model studies and in carotid angiography. During pulsatile flow, pulse-synchronous changes of contrast density can be registered. The maxima of opacification coincide with the doppler-sonographic phase of re-increasing blood flow after a preceding depression. This is caused by the accumulation of contrast medium at the tip of the catheter during diastole and the subsequent dilution of contrast medium by the increasing blood flow during systole. In model studies as well as in angiography, the time interval between the doppler-sonographic maximum of flow and the maximum of contrast density is very constant and is prolonged with increasing distance from the tip of the catheter. This latency between 2 maxima which are registered at the same region of interest, is caused by the higher speed of propagation of the flow wave compared to the actual movement of the blood corpuscles and contrast medium. The understanding of the relationship between cardiac action, blood flow, and contrast density may improve the functional interpretation of angiograms.