This is a general discussion of the problem of protein utilization in tissue synthesis. The roles of protein stores and of protein interchange between plasma and tissues are considered in relation to amino acid utilization. The literature reviewed and the data presented indicate that tissue proteins and dietary proteins intermingle freely and dynamically in the process of intermediary metabolism and that homeostatic mechanisms, which presumably operate in an effort to maintain a constant internal environment, tend to keep the metabolic processes in a state of "dynamic" equilibrium.