In this study, the structural components of mouse spleen were compared during their aging processes. Splenic tissues of 1 week-, 5 weeks-, 8 weeks-, 6 months-, 12 months-, 18 months-, 24 months-, and 30 months-old ICR mice were dissected out under anesthesia. Pieces of the tissues were taken from the spleen, fixed in 10% neutral formalin for light microscopy, and some splenic tissues, were prefixed with 2.5% glutaraldehyde-1.5% paraformaldehyde followed by post-fixation with 1% osmium tetroxide for electron microscopy. Paraffin sections stained with hematoxylin-eosin, Masson's trichrome, Bielschowsky's silver impregnation or aldehyde-fuchsin, were observed with light microscope. Ultrathin sections stained with uranyl acetate and lead citrate were observed with a JEM 100CX-II electron microscope. The observed results were as follow: 1. The thickness of the splenic capsule was prominently increased between 1 week-old and 5 weeks-old ones, whereas after 5 weeks-old, the thickness was very slightly increased during aging. 2. In the 1 week-old, 5 weeks-old and 8 weeks-old mice, blood forming cells were observed more frequently than those found in older ones. 3. The collagenous fibers and elastic fibers were increased in the spleens of 12 months-old mice, whereas after 18 months-old, fibers were not increased during aging. 4. The reticular fibers were increased by 8 weeks, whereas fibers were not increased afterwards. 5. In the 1-week old, mast cells were observed frequently, whereas from 5 weeks to 6 months they were observed rarely, and after 12 months, mast cells were observed frequently. 6. In the 5-weeks old, distended cisternae of granular endoplasmic reticula were first observed within the plasma cells. 7. After 12 months, the mast cell containing phagocytosed debris were observed frequently. 8. After 18 months, the plasma cell containing irregular distended cisternae of granular endoplasmic reticula were observed frequently. 9. In 30 months, the plasma cells containing myelin figures were observed frequently. From the above results, it was suggested that spleen of the mouse matures structurally in five weeks, but the function of the spleen is suppressed around 18 months, and thereafter the functional suppression is continued on aging.