Glutamate plays an important role in supraspinal nociceptive systems. Thus, glutamate is present in the nucleus submedius of the medial thalamus, a major relay for nociceptive information. In this study, immunoreactivity for the four subunits (GluR1-4) of alpha-amino-3-hydroxy-5-methyl-4-isoxasoleproprionate (AMPA) receptors was examined by a preembedding immunohistochemical method in order to evaluate the presence of this glutamate receptor subtype in the nucleus submedius. Combining the preembedding method with a postembedding immunogold technique, we found that AMPA receptor-like immunoreactivity was present postsynaptically to glutamatergic terminals but not to terminals containing gamma-aminobutyric acid (GABA). These findings suggest a role for AMPA receptors in excitatory synaptic transmission in the nucleus submedius of the rat thalamus.