Experiments have been made to determine which organ takes over the function of the thymus in polysaccharide metabolism if the latter undergoes involution and to study the effect which the morphological changes, undergone by the thymic structure in the course of life, produce upon its capacity to produce mast cells. It has been found that it is the lymph node which takes over the function of producing mast cells in old animals. Although the intensity of the paradoxical effect of cortisone varies in thymectomized animals of different ages, the effect itself appears in young, adult and aged animals alike. The role of processes of differentiation and dedifferentiation in the production of mast-cell reaction has also been analyzed.