Forty young female rats, aged about 3 months, were weighed on entry into the study; half were subjected to initial splenectomy. Animals were weighed at monthly intervals therafter. Ten animals from each group were killed at four months, and ten from each group at one year. Thymuses and spleens were dissected out and weighted. Because of progressive weight gain, all groups showed declining thymic index. Absolute thymus weight did not change signficiantly in splenectomized animals. Intact animals showed significantly reduced thymus weight at 4 months (P less than .001) and at 12 months (P less than .005). We conclude that the spleen may be a causative factor in the thymic involution of increasing age. The process is gradual and indolent compared to the rapid thymic weight depletion that occurs in the presence of growing tumor.