Thymocyte differentiation progresses through well-defined stages in which apoptosis is central to the selection of a functional TCR repertoire. In the present study, we explored the developmental effects of BCL-XL, a repressor of apoptosis. We found that endogenous BCL-XL is down-regulated by both positive and negative selection signals at the CD4+ CD8+ stage of thymocyte development. We examined the role of BCL-XL regulatable apoptosis in T cell development in the context of an alpha beta TCR transgene, Rag-1 deficiency and the scid model. We found that BCL-XL expression promoted accumulation of CD8 single-positive thymocytes even in MHC class II-restricted TCR transgenic mice. However, the apoptotic resistance conferred by BCL-XL could not fully substitute for TCR-mediated positive selection signals nor did it prevent negative selection. Overexpression of BCL-XL promoted partial maturation of CD4-CD8- thymocytes to CD4+CD8+ cells in a Rag-deficient, but not a scid background. Thus, TCR-mediated signals mediate the regulation of endogenous BCL-XL during thymocyte development, indicating that the principal protection of double-positive thymocytes by BCL-XL occurs prior to selection. The impact of BCL-XL on the CD8 but not CD4 lineage supports the asymmetric model of lineage commitment. Moreover, several critical control points in T cell development could be distinguished as BCL-XL responsive or unresponsive.