Fibrillar components of the thymus lobules have been studied in persons 9-73 years of age and in 11-30 month-old noninbred white rats. Beginning from 9-10 years of age, i. e. before the onset of age involution, in the medullary substance of the lobules argyrophil carcass is formed; it includes numerous microvessels. At the age of 30-40 years a part of argyrophil fibers is subjected to destruction. At the same time in the medullary substance adipocytes appear. Their increasing amount is accompanied with substitution of the medullary substance for adipose substance, while a part of the cortical substance is preserved. These data are confirmed, when the thymus in old animals is studied. Ultrastructural manifestations of extra- and intracellular destruction of fibers are noted in them, too. The dynamics of the fibrillar component changes in the thymus in different animal individuals is not the same and can serve as one of the criteria of its age involution.