The question naturally arises in the mind as to whether a diet may be satisfactory for the maintenance of optimal nutrition in the adult, but unsuited to the nutritive needs of the young. We have sought to gain an answer to this question. By restricting youthful adult animals to diets which were known to be incapable of supporting growth in the young, we have convinced ourselves that there is no important difference in the nutritive requirements of the young and the adult. Any diets which we have studied, which were not satisfactory for the promotion of growth in a young rat, were found to cause some damage to adults which were restricted to them. This might be manifested in early aging, short life, lowered fertility or the deterioration of a family restricted to it through several generations. We are of the opinion, therefore, that any diet which is not suited for the promotion of optimum nutrition during growth will fail to support an animal so as to induce a long span of life, and sustain the optimal well-being over a long period.