The data of Osborne and Mendel discussed above, have an interest quite apart from their worth as quantitative comparisons of the relative values of proteins. They support the view that "Economy of food can be effected only by supplying the young animal with as much as it will eat; economy of protein only by reducing the nutritive ratio below that at which the normal rate of growth can be maintained." This principle appears to be established, but it is quite another matter to determine whether or not economy of protein utilization obtained by the means just mentioned, makes for physiological well-being. The studies of Mc-Collum and Simmonds, to be described later, point to the desirability, from the standpoint of the long maintenance of vigor, of a liberal supply of protein in a diet which is otherwise satisfactorily constituted with respect to all other factors.