Multiple-site optical recording of transmembrane potential activity, using a voltage-sensitive dye, was employed to monitor neural activity from the nucleus tractus solitarius of the chick embryo. Optical signals related to glutamate-mediated excitatory postsynaptic potentials were evoked by a brief square current pulse applied with a microsuction electrode to the vagus nerve, and were recorded simultaneously from many sites in the brainstem slice preparation. We have found that glycine has biphasic modulatory effects on the glutamate-mediated excitatory postsynaptic potentials: at lower concentrations, glycine enhances the glutamate-mediated excitatory postsynaptic potential-related optical signal, and at higher concentrations, it reduces the glutamate-mediated excitatory postsynaptic potential-signal. The enhancing effect was insensitive to strychnine, but the reducing effect was blocked by strychnine, suggesting that the former effect was induced by glycine which increased glutamate binding to N-methyl-D-aspartate receptors, and the latter resulted from an increase in chloride conductance through the strychnine-sensitive inhibitory glycine receptors in postsynaptic neurons. The inhibitory effect of glycine was first observed in the brainstem preparations at the seven-day-old embryonic stage, and the enhancing effect was first observed in the nine-day-old preparations. We determined regional distributions of the biphasic effects of glycine in the seven- to nine-day old embryonic preparations. The spatial distribution of the enhancing effect appeared to be concentrated on the ventral side of the nucleus tractus solitarius, and the inhibitory effect was relatively concentrated in the medial portion. Furthermore, we compared the glycine effect with the effect of Mg(2)+ on N-methyl-D-aspartate receptors, and we have found that the Mg(2)+ site is functionally organized prior to the glycine site during embryonic development.