We examined initial expression of synaptic function in the embryonic chick trigeminal nucleus using voltage-sensitive dye recording. Brainstem preparations with three trigeminal nerve afferents, the ophthalmic (V1), maxillary (V2) and mandibular (V3) nerves, were dissected from 5.5- to 6.5-day old chick embryos. In our previous study (Sato et al., 1999), we detected slow signals corresponding to glutamatagic EPSPs and identified the principal sensory nucleus of the trigeminal nerve (Pr5), spinal sensory nucleus of the trigeminal nerve (Sp5) and trigeminal motor nucleus (Mo5). In this study, we examined effects of removing Mg²⁺ from physiological solution, which enhanced NMDA receptor function in the sensory nuclei. In 6.5-day old embryos, the slow signal was observed in the Pr5 and Sp5 only when the V1 nerve was stimulated, whereas it appeared in Mg2+-free solution with every nerve stimulation. In 6-day old embryos, the slow signal was observed only in the Sp5 with the V1 nerve stimulation, and the appearance of synaptic function in Mg²⁺-free solution varied depending on nerves and preparations used. In 5.5-day old embryos, synaptic function was not detected even when external Mg²⁺ was removed. These results indicate that the initial expression of synaptic function in the trigeminal system is earlier than previously considered, and that the developmental organization of synaptic function is different between the three trigeminal nerves and between the two sensory nuclei.