T cells expressing the RT6 surface alloantigcn perform important immunoregulatory functions in the rat. Diabetes prone (DP) BB rats arc deficient in circulating RT6+T cells and develop spontaneous autoimmune diabetes mellitus. Transfusions leading to cngraftment of RT6′T cells prevent the disease. Coisogcnic diabetes resistant (DR) BB rats do circulate RT6^ T cells and are free of disease. We investigated the basis for the deficiency of RT6 * T cells in the DP-BB rat and made the following observations. 1. Thymectomy causes the rapid loss of most peripheral T cells in the DP-BB rat. 2. Concomitant with the loss of T cells is the total loss of mRNA encoding RT6. 3. In contrast to the effects observed in peripheral lymphoid tissues, thymectomy does not lead to a detectable loss in RT6+protein found in the small intestine. We conclude that the deficiency of RT6+peripheral T cells in the DP-BB rat is due either to their short life span or to their reduced proliferative capacity following release from the thymus.