A strongly positive alkaline phosphatase reaction in endothelial cells of capillaries and some precapil laries and a considerable activity of α-naphthylacetate esterase in endothelial cells of postcapillary vessels allow a complete demonstration of vasa vasorum in arteries of the muscular type.This method gives results also in such parts of arteries or pieces of their wall which do not allow a demonstration of their vasa vasorum by means of injection methods. In the internal spermatic artery of the bull a dense rope-ladder shaped capillary network is demonstrable within the outer parts of the tunica media with the alkaline phosphatase and the α-naphthylacetate esterase reactions. The two methods also may be combined. Most of the vasa vasorum arise directly from the lumen of the artery. They set through the whole of the arterial wall and spread over the vessel with retrograde branches. A uniformely thick inner portion of the arterial wall has no capillarie