N-Methyl-D-aspartate receptors are present in the nodose ganglion, which contains the cell bodies of vagal afferents, and in the nucleus tractus solitarius, where these afferent fibers terminate. This suggests that N- methyl-D-aspartate receptors are located presynaptically on visceral vagal afferents and/or their target neurons in the nucleus tractus solitarius. To test this hypothesis, we combined anterograde transport of biotinylated dextran amine, following injections into the left nodose ganglion, with electron microscopic immunogold labeling of antipeptide antiserum against the R1 subunit of the N-methyl-D-aspartate receptor in the nucleus tractus solitarius of rat brain. Within the medial nucleus tractus solitarius, the N- methyl-D-aspartate receptor R1 immunoreactivity was seen in dendrites (39% of 639 profiles), axons and axon terminals (41%), and a few neuronal perikarya and glia. Many vagal afferent axons and terminals (40% of 468 profiles) contained N-methyl-D-aspartate receptor R1 immunogold labeling. In addition, 42% of the dendrites contacted by vagal afferent terminals (n = 206) contained N-methyl-D-aspartate receptor R1 immunoreactivity. In axons and dendrites, the gold particles were occasionally seen within asymmetric postsynaptic junctions or at non-synaptic sites on the plasma membrane. More commonly, however, N-methyl-D-aspartate receptor R1 labeling was seen on membranes of vesicular cytoplasmic organelles, suggesting that there is abundant N-methyl-D-aspartate receptor protein available for activity- dependent mobilization to the plasmalemma. Since many vagal afferents are glutamatergic, our results implicate N-methyl-D-aspartate receptors in autoregulation of the presynaptic release and postsynaptic responses to glutamate at the level of the first central synapse in the nucleus tractus solitarius.