In these suggestions we have not theory only, but a number of very important facts which plainly must have some significance. Take, for example, the excretion of creatinin. It is a characteristic nitrogenous waste product, but its elimination from the body is wholly independent of quantitative changes in the total amount of nitrogen excreted. In other words, the amount of creatinin eliminated is a constant quantity for a given individual under ordinary conditions, no matter how great the variation in the amount of proteid food, provided no meat is eaten. Meat must be avoided in testing this point, since meat contains a certain amount of creatin, or other components, which would be excreted as creatinin. Further, it is found that every individual has his own specific creatinin excretion, which fact again emphasizes the idea that this substance is a product of true tissue katabolism, having no connection with that variable metabolism, of which urea is the striking representative. These are facts which cannot be ignored. They are well established by the careful observations of Folin, and they are confirmed by a large number of observations made in our own laboratory. Turn now to that other, more conspicuous, product of proteid katabolism, urea. With a so-called average proteid intake, about 88–90 per cent of the excreted nitrogen will be in the form of urea, but, as Folin states, “with every decided diminution in the quantity of total nitrogen eliminated, there is a pronounced reduction in the per cent of that nitrogen represented by urea. When the daily total nitrogen elimination has been reduced to 3 grams or 4 grams, about 60 per cent of it only is in the form of urea.” Here, we have the chief product of exogenous metabolism, a substance quite distinct from creatinin, just as the process by which it originates is likewise quite distinct.