The recombination activating genes, RAG-1 and RAG-2, are likely to encode components of the V(D)J site-specific recombination machinery. We report here the detection of low levels of the RAG-1 transcript in the murine central nervous system by polymerase chain reaction, in situ hybridization, and Northern blot analyses. In contrast, an authentic RAG-2 transcript could not be detected reproducibly in the central nervous system. The RAG-1 transcript was found to be widespread in embryonic and postnatal neurons, with transcription being most apparent in regions of the postnatal brain with a high neuronal cell density (the cerebellum and the hippocampal formation). The results suggest that RAG-1 functions in neurons, where its role might be to recombine elements of the neuronal genome site-specifically, or to prevent detrimental alterations of the genome in these long-lived cells.