To examine whether there are any differences in functional organization between the glossopharyngeal nerve (N. IX)- and vagus nerve (N. X)-projecting areas in the nucleus of the tractus solitarius (NTS), we performed optical recording of neural responses evoked by N. IX stimulation in 5- to 9-day old embryonic chick brainstem preparations, and compared the results with those in our previous studies concerning the N. X-related NTS. First, we investigated _DL-2-amino-5-phosphonovaleric acid (APV)-/Mg²⁺-sensitivity of the glutamatergic excitatory postsynaptic potentials (EPSPs) in the N. IX-related NTS. In 7- to 9-day old preparations, we found regional differences in the degree of both the APV-induced reduction and Mg²⁺ free-induced enhancement of the EPSPs. We constructed developmental maps of spatial patterns of the APV- and Mg²⁺-sensitive components, and showed that functional expression of the N-methyl-D-aspartate (NMDA) receptor dynamically changed during development. Second, we studied initial expression of synaptic functions in the N. IX-related NTS. In 6-day old preparations, although action potentials alone were usually detected in normal Ringer's solution, small EPSPs were elicited in a Mg²⁺-free solution. This result suggests that the NMDA receptor-mediated synaptic function is latently generated in the N. IX-related NTS at the 6-day old embryonic stage, and that external Mg²⁺ regulates the onset of synaptic functions. Developmental patterns of APV-/Mg²⁺-sensitivity and the stage of initial expression of the glossopharyngeal EPSP were similar to those of the N. X, suggesting that the developmental sequence of the synaptic function in the NTS is the same for the N. IX- and N. X-related NTS.