Neonatal mice were given a subcapsular, intrathymic injection of thymidine-3H using a modified microneedle technique, and the migration of labeled cells to spleen, lymph nodes, Peyer's patches, and bone marrow was followed radioautographically with time. Assuming that nonlabeled lymphocytes migrated in the same manner as labeled lymphocytes, it can be concluded that the majority of lymphocytes present within mesenteric lymph nodes (74%) and Peyer's patches (61%), and a large proportion of those located in popliteal lymph nodes (40%) and the spleen (26%), were of thymic origin. Evidence is presented indicating that these are minimum values. The difference in the magnitude of thymic cell migration to gut-associated lymphoid tissue on the one hand and to the spleen and popliteal lymph node on the other hand was tentatively attributed to antigenic stimulation from the intestinal flora which develops during the first days of life. Thymus-derived lymphocytes were scattered throughout the lymph node cortex and splenic follicles. No noticeable thymic cell migration to the bone marrow was found. Labeling indices in the peripheral lymphoid organs paralleled those of cortical thymic lymphocytes suggesting the thymic cortex as the major source of migrants. By 2 days postinjection, the mean grain counts of labeled lymphocytes in all peripheral lymphoid tissues were higher than the mean grain counts of labeled lymphocytes in the thymus. At 7 days postinjection heavily labeled cells constituted 11–16% of the labeled population in peripheral tissues while they were absent from the thymic cortex. These results indicate that a fraction of thymus-derived cells, upon settling in the periphery, remained in, or reentered, a nonproliferative phase for at least 7 days. Conversely, many thymus-derived lymphocytes underwent division in the periphery and/or penetrated the intestinal epithelium. Since the relative number of thymus-derived cells found in the mesenteric lymph nodes of 1- and 2-day old mice was considerably higher than the percentage of cells at this site having the theta (θ) alloantigen, as reported by other authors, the possibility exists that θ-antigen on thymus-derived lymphocytes may, at least in a fraction of these cells, no longer be detectable as they reach the peripheral organs.