The main thesis of this paper is the "dynamic equilibrium" between body protein and plasma protein. A steady state or balance exists between the body protein stores, protein wear and tear, and protein production. Protein production includes the plasma proteins which probably represent the largest fraction of new formed protein. Body protein reserve stores are largely intracellular. We believe the plasma proteins are the means of a fluid interchange between reserve stores and the organ cells in which protein is produced, modified, and utilized in the body economy. The term "protein pool" suggests this fluid exchange within the body.Earlier experiments in this laboratory showed that plasma protein as plasma as the sole source of protein given by vein could maintain nitrogen equilibrium for 2 to 4 weeks (3, 11, 13). The last report (3) showed evidence of some "intoxication" which developed in two of these experiments. An attempt to reproduce this "intoxication" in the experiments tabulated below was not successful, and we are inclined to explain the "intoxications" previously described. as due to contaminated plasma or vitamin deficiency or both. The experiments given below are quite satisfactory and diet deficiencies were guarded against. The plasma was handled with more complete asepsis so we conclude that "intoxication" as described (3) is not an inevitable part of the experiment when large volumes of dog plasma are givenparenterally.In several experiments in other series at various intervals the dog's circulating plasma was fractionated and various elements estimated after the parenteral injection of plasma had been started--e.g., fibrinogen, globulins, albumins using coagulation, chemical methods, and electrophoretic technique. No uniform deviation from the normal plasma protein pattern was observed. Depleted dogs have a tendency to show some increase in globulins at the expense of albumins. It seems to us that the behavior offibrinogen is a very good illustration of the fact that long continued plasma given over months does not modify significantly the level of any plasma protein in the circulation. Fibrinogen is a very special protein used in the body for the production of blood clots. Yet obviously it is used in the body economy presumably within ceils to supply special cell needs (other than coagulation) since otherwise the fibrinogen