The possibility that damaged neural circuitries can be repaired through grafting has raised questions regarding the cellular mechanisms required for functional integration of transplanted neurons. Invertebrate models offer the potential to examine such mechanisms at the resolution of single identified neurons within well-characterized neural networks. Here it is reported that a specific deficit in the respiratory behavior of a pulmonate mollusc, caused by the ablation of a solitary interneuron, can be restored by grafting an identical donor interneuron. The transplanted interneuron not only survives and extends neurites within the host nervous system, but under specific conditions forms synapses with appropriate target neurons and is physiologically integrated into the host's circuitry, thereby restoring normal behavior.