The researches of the past fifteen years on the chemistry and biochemistry of creatin and creatinin have abundantly established the high importance of these substances in the animal economy, and it seems appropriate at the outset of this paper to present a brief summary of those investigations which have a direct bearing on the subject here considered. Previous to 1906, general acceptance was given to the theory of the older investigators1that the creatinin eliminated in the urine is a waste product derived from muscle creatin. In 1906 Folin published the results of experiments2in which he showed that ingestion of creatin did not cause the appearance of creatin in the urine, and did not appreciably influence the elimination of creatinin, from which he concluded that though chemically closely akin, creatin and creatinin are biologically distinct. Since that time numerous observers have confirmed these experiments (Klercker, Wolf, Shaffer) and the