Functional organization of the brainstem vestibulo-cochlear nuclei during embryogenesis was investigated using a multiple-site optical recording technique with a fast voltage-sensitive dye. Brainstem slices with the cochlear and/or vestibular nerves attached were dissected from 6- to 8-day old (E6 to E8) chick embryos. Electrical responses evoked by cochlear or vestibular nerve stimulation were optically recorded simultaneously from many loci of the preparations. In E7 and E8 preparations, we identified two components of the optical response with cochlear or vestibular nerve stimulation; one was a fast spike-like signal related to the action potential, and the other was a slow signal related to the glutamate-mediated excitatory postsynaptic potential. The location of the cochlear nerve response area was mainly located on the dorsolateral region, while that of the vestibular nerve was deviated ventro-medially. At E6, cochlear nerve stimulation evoked only the fast spike-like signals in normal Ringer's solution. However, when we removed Mg²⁺ from the extracellular solution, significant slow signals were elicited in the E6 preparation. The present results demonstrated that in the chick vestibulo-cochlear nuclei functional synapses are already generated by the E7 embryonic stage, and that postsynaptic activity related to NADA receptors emerges latently, at least in the cochlear nerve-related nucleus, at the E6 embryonic stage. This chronological sequence of the emergence of postsynaptic function is different from that reported previously (E10-E11), suggesting that the developmental origin of sensory information transfer in the auditory pathway is much earlier than has been anticipated.