The protein encoded by the rat brain cDNA 1B236 has been shown to be identical to myelin-associated glycoprotein (MAG). In this report we describe the cellular distribution of 1B236/MAG mRNA transcripts in rat brain by using in situ hybridization. At postnatal day 20, large numbers of 1B236/MAG mRNA-containing oligodendrocytes are concentrated in myelinated fiber tracts and throughout gray matter regions. The presence of high levels of 1B236/MAG mRNA within oligodendrocytes at postnatal day 20 is consistent with the proposed role of MAG in formation of the myelin sheath during development. In the adult brain, our results suggest that not only is 1B236/MAG mRNA expressed at reduced levels within oligodendrocytes but also 1B236/MAG or a 1B236/MAG-like mRNA is present within neurons. This localization is consistent with the results of previous immunocytochemical studies using antibodies against the 1B236/MAG mRNA with different cell-type-specific patterns of expression suggests that oligodendrocytes and neurons employ different mechanisms for regulating the same gene. Thus, different cell types may use a similar cell adhesion molecule both during myelinogenesis and in the mature nervous system.