The recorded observations are interpreted on the basis that part at least of the protein in the tissue is maintained by a metabolic system in which synthesis and breakdown of protein occur simultaneously, and that these two processes are not two phases of a reversible system but are composed of two separate groups of reactions. The data obtained indicate that protein breakdown is controlled by an iron-containing enzyme system which may well involve cytochrome and that this is associated with the Krebs cycle of organic-acid metabolism. The synthesis of protein appears to be controlled by a copper-containing enzyme system which in the swede ‘root’ may well involve ascorbic acid. C4 and C6 organic acids are utilized in the process of protein synthesis.