In order to investigate the aetiology of Perthes' disease in the dog the author has conducted a comparative anatomical study of the vascular system of the femoral heads in miniature dogs and in normal-size mongrels. The study was made in five three-months old miniature dogs and five agematched normal-size mongrels, in which the epiphyseal plate of the femoral head was still open. The most distinct difference between the two species was in the channel of the superior retinacular vessels. In miniature dogs, these vessels go through the shallow neck and appear as a “suspended bridge”. In normal-size mongrels they go through the deep fossa of the femoral neck and appear to be very rigidly stable. Although foveolar vessels in the round ligament were present in both species, no vessels were found in either species reaching the epiphysis by penetrating the articular cartilage at this stage of development.