Abstract— —Twenty seven dogs (two of them obtained by inbreeding) in which Legg-Perthes' disease was verified clinically and roentgenologically, were studied to elucidate the early changes encountered. Histological-histochemical methods, oxytetracycline (OTC) bone labelling and auto- and micro-radiography techniques were used. All of the twelve young dogs with unfused growth plates showed histologically and histochemically, especially in the proximal part of the epiphysial cartilage, areas filled with a structureless, metachromatically staining mass, cyst-like formations and derangement of the architecture as the initial changes of the disease. By using S35 it was possible to confirm the histochemical findings that substances belonging to the mucopolysaccharide group were formed in the degenerated areas of the growth plate. Histologically, degenerative changes were demonstrable earlier in the metaphysis than in the epiphysis. Absence of OTC uptake preceded bone necrosis histologically. Oxytetracycline bone labelling showed that regeneration derived chiefly from the metaphysis as the epiphysial cartilage began to disappear. Using microradiography, it was possible to illustrate the bone structure, especially to reveal the flattening of the epiphysis and the broadening of the metaphysis. In the later phase of the disease extensive articular cartilage changes were recorded. Together with the other degenerative changes described, they contributed to the formation of secondary osteoarthritis.