Little is known concerning the mechanisms which control the concentration of the various proteins in the serum of blood. The authors found interesting changes in the concentration of serum proteins in the blood of patients with lesions in the hypothalamic region. The changes could not be attributed to malnutrition or disturbance of the pituitary gland. The change in the serum proteins consisted in a decrease in the concentration of albumin followed by an increase in the concentration of globulin. The total concentration of serum proteins was not altered and electrophoresis did not reveal the change to be due to any one fraction. There were no alterations in the concentration of electrolytes or urea in the blood. The authors suggest that the evidence indicates a specific role of the hypothalamus in regulating concentrations of the serum proteins in blood, but the nature of the lesions did not permit more accurate localization of the portion of the hypothalamus which may have this function. The serum proteins were studied in patients suffering from a variety of other intracranial lesions but no abnormality was found.