THE EFFECT OF FASTING ON THE SERUM PROTEIN CONCENTRATION OF THE RAT : WITH SPECIAL REFERENCE TO THE QUESTION OF THE EXISTENCE OF AN IMMEDIATELY UTILIZABLE CIRCULATING PROTEIN FRACTION.

1. Experiments were carried out to test the hypothesis that there exists a special circulating fraction of plasma protein available for use by the tissues. 2. The changes in serum protein concentration after varying periods of fasting were followed in large numbers of individual rats. 3. Previous reports from this laboratory of a small initial drop in the total protein concentration of the serum, with subsequent maintenance of the serum protein at the new level are confirmed. 4. Evidence is offered that this initial fall involves chiefly or solely the albumin fraction. 5. The mechanism responsible for the observed initial drop and subsequent maintenance of the protein is not exactly known, but two definite factors are age and individual resistance. 6. It is concluded that no satisfactory evidence is available to support the hypothesis of a directly utilizable protein fraction in the blood.