T cell receptor (TCR) signaling mediates cell fate decisions throughout the life of a T cell. The earliest biochemical events during antigen-stimulated TCR signaling include activation of the Src-family protein tyrosine kinase, p56 Lck (Lck), which is an integral component of the TCR signaling complex by its association with the cytoplasmic tails of CD8 or CD4. CD8 and Lck are obligatory during thymic selection of CD8 + T cells. What remain unknown are when and with what stringency Lck is required for effective TCR-mediated activation and function throughout the life of a mature CD8 + T cell. Using mice that express an inducible Lck transgene in T cells, we have investigated the temporal importance of Lck-mediated TCR signaling in antigen-specific CD8 + T cell responses during acute viral infections. We show that Lck deficiency induced in naive mice abrogated the antigen-specific activation and clonal expansion of CD8 + T cells during a primary response to acute viral infections. Moreover, the magnitude of primary CD8 T cell expansion depended on the duration of Lck-dependent TCR signaling. Quite unexpectedly, however, Lck was dispensable for enhanced functional avidity, maintenance, and reactivation of memory CD8 + T cells in vitro and in vivo . These observations suggest that the TCR signaling apparatus is rewired from an Lck-dependent state in naive CD8 + T cells to an Lck-independent state in memory CD8 + T cells. Less stringent requirements for antigen-specific TCR signaling to activate memory CD8 + T cells could, in part, account for their unique hyperreactivity to antigen, which contributes to accelerated immune control during secondary infections.