After general remarks on the flow behaviour in blood circulation some experiments are described which were carried out on models in the Aero dynamischen Institut at the TH Aachen. In artery branchings, especially in prostheses, the sedimentation of substances which cause a stenosis can become critical because of secondary flows. Results obtained up to now are described on the variations of the unsteady flow conditions caused by the elasticity of the artery walls, as well as on the possibility and the danger of gas separation induced by the flow and wave phenomena. On models of artificial heart valves (aorta and mitralis prostheses) the flow pattern is observed and the pressure drop is measured; here the charac teristic parameters (size of prostheses related to aorta diameter, lift of the valve body, asymmetric position in the aorta inlet) are systematically varied, From the drag measurements an optimum arrangement for each valve type can be evaluated concerning the reduction of the heart muscle performance and of the formation of stagnation regions (danger of thrombosis). Finally model tests with swine blood in a special facility have confirmed that certain flow phenomena bring on a mechanically caused hemolysis."