Granulomas were induced in rats of both sexes at different weeks of age by subcutaneous implantation of formalin-soaked filter-paper disks. The sensitivity changes with aging to the antigranuloma and thymolytic actions of prednisolone acetate were studied by measuring the dry-defatted weight of granulomas and the wet weight of thymus. The prednisolone acetate effects on these tissues were also investigated by electron microscopy. In male rats, the amount of granulation tissue increased from 6 to 12 weeks of age; but no difference in granuloma weight was evident between the 12- and 20-week-old groups. In females, the granuloma weight also increased form 6 to 8 weeks. However, heavier granulomas were found in males than in females. In both sexes, the 20-week-old group showed lower sensitivity to the antigranuloma and thymolytic actions of prednisolone acetate than the 6- and 8-week-old groups. In the 6-, 8-and 20-week-old groups of both sexes, marked differences were not found between the antigranuloma potency and thymolytic potency of prednisolone acetate. The prednisolone acetate dose required to produce appreciable electron microscopic changes were about the same in granulation tissue and thymus. The present results show that rat granulation tissue and thymus have similar degrees of sensitivity to glucocorticoids and that the sensitivity of these tissues is lowered with aging.