Basic fibroblast growth factor-responsive neural stem cells (NSCs) derived from adult rat hippocampus were earlier demonstrated to generate neurons and glia. These stem-cell-derived neurons express GABA, acetylcholinesterase, tyrosine hydroxylase, or calbindin. It has not been clear, however, whether or not these stem-cell-derived neurons are able to form functional synapses. In the present study, we investigated the development of synapse formation by adult hippocampus-derived neural stem cells. NSCs from adult rat hippocampi and primary embryonic rat hippocampal neurons were cocultured on a glial feeder layer. Immunofluorescence studies revealed that some of the NSCs became immunoreactive for microtubule-associated protein 2ab, neurofilament 200, synaptobrevin, or synaptophysin. These cells possessed properties of functional neurons such as action potentials and miniature postsynaptic currents (mPSCs). The elicited mPSCs with rapid kinetics were blocked by 6,7-dinitroquinoxaline-2,3-dione (DNQX), but not by bicuculline (excitatory mPSCs). The remaining mPSCs had slower kinetics and were blocked by bicuculline, but not by DNQX (inhibitory mPSCs). We considered that the neurons derived from the adult NSCs expressed both non-NMDA glutamate receptors and the GABA_A receptors and formed functional synapses. Our results demonstrate that adult NSCs can differentiate into neurons with functional glutamatergic and GABAergic synaptic transmission in vitro and support the concept that such neurons could integrate into the neuronal circuitry.