The question of a possible effect exerted by insulin on protein metabolism is of considerable significance both from a metabolic and a clinical standpoint. Intermediate carbohydrate metabolites are capable of synthesis together with nitrogenous compounds into protein. The well known sparing effect of carbohydrate on protein breakdown is probably due in part to such anabolic processes, as isodynamic amounts of fat fail to exert this influence. If, therefore, it could be shown experimentally that insulin increased this sparing function of carbohydrate, the conclusion would be near that this hormone probably had played a definite rôle in the synthesis of protein from carbohydrate. To test this hypothesis, a short series of metabolic experiments were conducted in an institution especially equipped for metabolic patients, insuring accuracy in the specimen collections and in the preparation, weighing and recording of the food. The fluid intake was distributed equally during the experimental periods. Normal human