The polycystic organ thought to be the thymic rudiment in "athymic" nude mice increased with age and continues its secretory activity even in senescent nude mice. Only the epithelial lining of the cysts undergoes pressure atrophy. Analogous polycystic structures can be found also in euthymic mice with or without the nu gene. A lymphoepithelial rudiment is found in the thymic area of nude mice; it may represent the true endodermal thymic rudiment and generate the T cells. In addition, a peculiar, sometimes enlarged and thymus-like node is present laterocaudally from the polycystic organ.