The messenger RNA expression of non-N-methyl-D-aspartate glutamate receptor subunits (GluR1-4), considered alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid type, was investigated in rat brain by in situ hybridization histochemistry using oligonucleotide probes specific to each subunit sequence. GluR1-4 subunit messenger RNAs were expressed widely and abundantly throughout the CNS. However, the combination of expression pattern varied notably according to location. GluR2 messenger RNA was expressed most strongly and widely, with most areas except the Bergmann glia containing this messenger RNA. GluR4 messenger RNA was also present widely, although the expression level was low. However, we observed many areas which lacked or expressed very little GluR1 messenger RNA, such as some nuclei in the general motor system and auditory system. In addition, some nuclei in the hypothalamus and general somatosensory system lacked or expressed very little GluR3 messenger RNA. These results suggest that in the rat CNS non-N-methyl-D-aspartate receptors varied their composition according to the area where they were expressed, and that the combination pattern might be related to the functional role of neurons.