This paper reports the fate of medical collagen after injected into rats. It is highly purified collagen. The persistence of it in rats is more than one year. It was found that fibroblasts and capillaries grew into the injections. Picrosirius-polarization demonstrated that the ration of type I and type III collagen changed. Fine green fibers of type III collagen were predominant at the early stage. Twelve months after implantation, most collagen fibers appeared as thick brilliant red fibers of type I collagen.