Activated T cells reach the lymph nodes via afferent lymphatics but it is unknown to what extent they also enter them directly via high endothelial venules (HEV). Little is known about the mechanism mediating the proliferation of activated T cells within lymphoid tissues in vivo or the subsequent fate of the progeny. Therefore, we stimulated rat T cells via TCR and CD28 in vitro and after injection identified them in the blood and the HEV of lymphoid organs at several time points. In addition, the proliferation of these cells was studied after entering different lymphoid organs. Our results show that, firstly, activated T cells continuously enter lymph nodes and Peyer's patches directly via HEV. Second, they proliferate within lymphoid organs, the rate significantly depending on the microenvironment. Third, mainly CD8+ progeny are able to leave the tissues and re-enter the blood. Thus, the distribution of activated T cells circulating through the body can be regulated during entry, but also within the tissue by influencing their proliferation and subsequent release.