Accumulating evidence has shown that lymphocytes modulate behaviour and cognition by direct interactions with the central nervous system. Studies have shown that reconstitution by adoptive transfer of lymphocytes from wild type into immune-deficient mice restores a number of neurobehavioural deficits observed in these models. Moreover, it has been shown that these effects are mostly mediated by T lymphocytes. Studies of adoptive transfer thus far have employed adult mice, but whether lymphocytes may also modulate behaviour during development remains unknown. In this study, neonate lymphocyte-deficient Rag2-/- mice were reconstituted within 48 hours after birth with lymphoid cells from transgenic donors expressing green fluorescent protein, allowing for their identification in various tissues in recipient mice while retaining all functional aspects. Adolescent Rag2-/- and reconstituted Rag2-/- along with C57BL/6J wild-type mice underwent a series of behavioural tests, including open field, social interaction and sucrose preference tests. At 12 weeks, they were evaluated in the Morris water maze (MWM). Reconstituted mice showed changes in almost all aspects of behaviour that were assessed, with a remarkable complete rescue of impaired social behaviour displayed by adolescent Rag2-/- mice. Consistent with previous reports in adult mice, neonatal reconstitution in Rag2-/- mice restored spatial memory in the MWM. The presence of donor lymphocytes in the brain of neonatally reconstituted Rag2-/- mice was confirmed at various developmental points. These findings provide evidence that lymphocytes colonize the brain during post-natal development and modulate behaviour across the lifespan supporting a role for adaptive immunity during brain maturation.