SOLUTIONS of macromolecular substances have been used for the purpose of expanding the volume of plasma by virtue of their colloidal osmotic activity while they remain in the blood. Because of the large size of their molecules, their diffusion from the blood is correspondingly reduced and their colloidal effectiveness prolonged. It is obvious that such substances do diffuse rather rapidly into the extravascular fluids and some are sequestered in specific tissue spaces. The rapid decline of their concentration in the blood after intravenous injection indicates that considerable portions have left the blood within a few minutes. Grotte, Knutson and Bollman1 found that dextran injected intravenously rapidly appeared in the lymph and that the colloidal osmotic gradient between the dextran in the blood and that in the extravascular fluids appear to be rapidly dissipated. The clinical effectiveness of these substances,2 however, appears to be more prolonged than would be