The demonstration of a preponderance of T cells in the thoracic duct lymph of rabbits prompted us to initiate cytokinetic studies using both uridine- and thymidine-labeled thoracic duct lymphocytes (TDL). Rabbits received intravenous injections of 8 to 11 X 10(8) autologous or allogeneic TDL, 95 per cent of which had incorporated 3H-uridine during a 1-hour in vitro incubation. Autoradiographs of tissues collected 24 hours after injection of TDL failed to demonstrate any trapping of label in liver or in vivo reutilization of 3H-uridine. No differences in the distribution of labeled cells were noted between recipients of autologous and allogeneic TDL. The paracortical areas of lymph nodes and periarterial areas of splenic follicles contained many heavily labeled cells; these areas therefore appear to correspond to thymus-dependent areas as in other rodents. The tonsils contained densely packed sheets of labeled cells. In the tonsillar but not in other germinal centers, evenly distributed, lightly labeled cells were seen. Small clusters of heavily labeled cells were seen in the bronchus-associated lymphoid tissue. In the appendix, sacculus rotundus, and Peyer's patches (GALT) densely packed, heavily labeled cells were seen in the interfollicular areas; fewer, heavily labeled cells were scattered throughout the dome and corona of GALT. In the dome and corona, however, there were many lightly labeled cells. The germinal centers of GALT were lacking totally in uridine-labeled cells. Some 20 per cent of TDL collected 24 hours after injection of uridine-labeled TDL were labeled, thereby reflecting considerable recirculation of TDL. Injection of 24 to 48 X 10(6) thymidine-labeled immunoblasts, obtained from TDL incubated with 3H-thymidine in vitro, into autologous or allogeneic recipients killed 24 hours later, revealed heavily labeled cells in the intestinal lamina propria, the dome, corona, and interfollicular areas of GALT, as well as throughout the spleen, lymph nodes, and tonsils. There was a 2- to 3-fold higher concentration of labeled cells in the appendiceal and mesenteric lymph nodes than in respective popliteal nodes. The potential significance of this differential distribution together with the homing-circulation patterns of these cells in GALT are discussed with regard to differentiation of IgA-producing cells. It is concluded that rabbit GALT contains a substantial number of cells belonging to the pool of recirculating lymphocytes and that B and T cells in the thoracic duct may have different rates of uridine incorporation as has been shown in other rodents.