How grafted neural stem cells (NSCs) and their progeny integrate into recipient brain tissue and functionally interact with host cells is as yet unanswered. We report that, in organotypic slice cultures analyzed by ratiometric time-lapse calcium imaging, current-clamp recordings, and dye-coupling methods, an early and essential way in which grafted murine or human NSCs integrate functionally into host neural circuitry and affect host cells is via gap-junctional coupling, even before electrophysiologically mature neuronal differentiation. The gap junctions, which are established rapidly, permit exogenous NSCs to influence directly host network activity, including synchronized calcium transients with host cells in fluctuating networks. The exogenous NSCs also protect host neurons from death and reduce such signs of secondary injury as reactive astrogliosis. To determine whether gap junctions between NSCs and host cells may also mediate neuroprotection in vivo, we examined NSC transplantation in two murine models characterized by degeneration of the same cell type (Purkinje neurons) from different etiologies, namely, the nervous and SCA1 mutants. In both, gap junctions (containing connexin 43) formed between NSCs and host cells at risk, and were associated with rescue of neurons and behavior (when implantation was performed before overt neuron loss). Both in vitro and in vivo beneficial NSC effects were abrogated when gap junction formation or function was suppressed by pharmacologic and/or RNA-inhibition strategies, supporting the pivotal mediation by gap-junctional coupling of some modulatory, homeostatic, and protective actions on host systems as well as establishing a template for the subsequent development of electrochemical synaptic intercellular communication.