Myelin-associated glycoprotein (MAG) is expressed on the surface of glial cells and is thought to act as a glial-neuronal adhesion molecule during early stages in the myelination process. Sequencing of several cDNA clones predicted the existence of two classes of MAG mRNAs which differ in the presence or absence of a 45 nucleotide insert near the 3' end. These two mRNAs are sufficient to encode the two MAG proteins previously described (p67MAG and p72MAG) and show that they differ only in their carboxyl terminal regions. The results of RNAse protection experiments reported here confirm the existence of two mRNAs for MAG which arise by alternative splicing of exon 12, as shown by Lai et al. Our results show that the p72MAG mRNA is expressed during the time of active myelin formation in the CNS, reaching a peak by post-natal day 22 and thereafter declining to adult levels by day 62. Conversely, p67MAG mRNA is produced as the minor species during myelin formation, but becomes the predominant form in adult brain. Cultures of oligodendrocytes express both forms of MAG. In the PNS, mRNA coding for p67MAG is predominant throughout development, reaching peak levels at day 6-10, whereas p72MAG mRNA is a very minor species. Alternative splicing also occurs at the 5' terminus. One form of mRNA lacking exon 2 from the 5' non-coding region is predominant in PNS, whereas mRNA containing exon 2 predominates in the CNS. Therefore, at least two and possibly four different mRNA species encode MAG. These results confirm the hypothesis that the two forms of MAG (p67 and p72) are generated by alternative splicing and show that each form is differentially regulated during development in the CNS and PNS.