In 1905 Folin (19) published three remarkable papers in which he made it clear that the process of the destruction of the greater part of the protein which is digested and absorbed daily, and of its conversion into the waste products which constitute the normal components of the urine, differs markedly from that type which is involved in the normal functioning of the tissues of the living body. The end-product of the first type is principally urea, while creatinin and uric acid are prominent components of the latter. When a diet free from protein or other nitrogenous compounds is taken in amount sufficient to cover the energy requirements of the body, the urine will, after a few days, contain about 60 per cent of its total nitrogen in the form of urea, 18 per cent as creatinin, 7.0 per cent as ammonia, and 3.4 per cent as uric acid. The remainder, which will amount to about 12-14 per cent of the total, is in forms which are in great measure uncharacterized, and which cannot as yet be estimated individually. A urine having this composition represents what Folin termed endogenous metabolism, to distinguish it from exogenous metabolism, which is concerned with the destruction of food protein that has never become a part of the living protoplasm of the body. The magnitude of the endogenous process is surprisingly constant, whereas that of the latter is very variable, and is determined by the consumption of protein. On ordinary planes of nutrition, exogenous metabolism or destruction of food is superimposed upon the endogenous or tissue waste type and the composition of the urine gives a composite picture of the two.