Auto-alloplastic arterial prostheses with a dacron mesh of 1 or 2 layers according to Sparks were investigated by light and electron microscopy after variable times of incorporation and perfusion. At the end of the organization-period the characteristic structure of the wall is composed of three layers. The interior layer is adapted to the form of the mandril, which holds the place of the lumen, the medial layer is formed according to the inforcing dacron-mesh. With increasing duration of incorporation the granulation tissue shows somewhat less inflammatory cells and capillaries and more collagen fibres. There were some characteristic disturbances of healing only partly due to infection, which resulted in focal or total necrosis of the granulation tissue: Not all of these findings were of clinical importance. Already after a perfusion time of 10 months an atherosclerosis of variable degree was found. This atherosclerosis started from mural thrombi and showed in the electron microscope proliferations of modified smooth muscle cells. Advantages and disadvantages of the autoalloplastic arterial prostheses are discussed.