The nonprotein nitrogen fraction of the blood has for some time been the object of considerable study. This is due partly to the recognized intimate bearing it has on protein metabolism of the tissues, particularly the problem of the fate of protein digestion in the body, and partly due to the publication of methods which made possible a much more accurate determination of this fraction than was heretofore possible. The studies and observations recorded so far are confined entirely to the blood of the human adult and of lower animals. To our knowledge, there is no reference in the literature on pediatrics to a study of the nonprotein fraction of nitrogen in the blood of the infant. The quantitative determination of this fraction and some of its components in the newborn at various ages and at different periods of digestion, was the object of our study. The difficulty in securing