Small infusions of dextran fractions having average molecular weights ranging from 10,600 to 412,000 yield plasma to lymph concentration ratios which are directly proportional to molecular weight. The concentration gradient for a specific molecular weight, however, decreases as the volume of infusion is increased. This volume effect, explained in terms of stretching of capillary pores, consequently provides less resistance to the passage of macromolecules through the capillary wall. The significance of these results in terms of the conventional pore theory of capillary permeability is discussed.