CD4(+)Vbeta5(+) T cells enter one of two tolerance pathways after recognizing a peripherally expressed superantigen encoded by an endogenous retrovirus. One pathway leads to deletion, while the other, termed TCR revision, results in cellular rescue upon expression of an alternate TCR that no longer recognizes the tolerogen. TCR revision requires the rearrangement of novel TCR beta-chain genes and depends on recombinase-activating gene (RAG) expression in peripheral T cells. In line with recent findings that RAG(+) splenic B cells are immature cells that have maintained RAG expression, it has been hypothesized that TCR revision is limited to recent thymic emigrants that have maintained RAG expression and TCR loci in a recombination-permissive configuration. Using mice in which the expression of green fluorescent protein is driven by the RAG2 promoter, we now show that in vitro stimulation can drive reporter expression in noncycling, mature, peripheral CD4(+) T cells. In addition, thymectomized Vbeta5 transgenic RAG reporter mice are used to demonstrate that TCR revision can target peripheral T cells up to 2 mo after thymectomy. Both sets of experiments strongly suggest that reinduction of RAG genes triggers TCR revision. Approximately 3% of CD4(+)Vbeta5(+) T cells in thymectomized Vbeta5 transgenic reporter mice have undergone TCR revision within the previous 4-5 days. TCR revision can also occur in Vbeta5(+) T cells from nontransgenic mice, illustrating the relevance of this novel tolerance mechanism in unmanipulated animals.