In this study we present evidence that gammadelta T cells are present in the normal mouse central nervous system (CNS). Compared with matching spleen gammadelta T cells, CNS gammadelta T cells expressed only the CD45RBlow phenotype, suggesting that CNS gammadelta T cells belong to the memory cell population. Approximately 20% expressed exclusively the CD8alphabeta heterodimer, consistent with a thymic origin. Gammadelta T cells in both spleen and CNS expressed higher levels of the IL-2rbeta (CD122), as well as Fas and FasL, than alphabeta T cells, suggesting that these cells function as immunoregulatory T cells. RT-PCR analysis showed almost exclusive use of Vdelta6 in the CNS whereas more Vdelta genes were expressed in the spleen. Sequencing of Vdelta6 RT-PCR products demonstrated a polyclonal population of T cells in the spleen but a more clonal population within the CNS. The predominant CNS sequence was found in all animals studied and was also detected in the spleen. From these data we conclude that a selective component of circulating gammadelta T cells traffics through the CNS. Thus, all major populations of lymphocytes can be detected in the normal CNS and as such may play specific roles in the immunological surveillance of that organ.