In this study we quantified CD8+ and CD4+ T cells in T lymphocytopenic BB rats as compared with control rats at given stages along the maturational pathway from immature thymocytes to mature peripheral T cells. Our results show that BB rats exhibit abnormal thymocyte subset distribution. Numbers of mature TCRhigh/CD4-8+ thymocytes, and also their TCRhigh/CD4+8+ precursors were decreased, as were levels of CD8 expression on all thymocyte subsets investigated. By analogy with mouse thymocyte development, these findings suggest a decreased efficiency for positive selection of CD8 precursors in BB rats. Furthermore, as related to the number of available mature TCRhigh single positive thymocytes, numbers of CD4+ and CD8+ T cells most recently migrated from the thymus were severely decreased in BB blood, indicating either reduced thymic output or rapid cell death after migration. Subsequently, in peripheral blood and cervical lymph nodes, a 95% decrease of CD8+ and a 50 to 80% decrease of CD4+ T cells were demonstrated upon maturation from recent thymic migrants to mature peripheral T cells, leaving the BB rat with a severely reduced T cell population, consisting of CD4+ T cells and a minute population of CD8+ T cells. The vast majority of the latter was found to have an immature peripheral phenotype. Possible consequences of our findings for the generation of autoreactive CD8+ T cells are discussed.