Two forms of myelin-associated glycoprotein (MAG) mRNA are produced by alternative splicing of the exon 12 portion. Expression of the two forms of MAG mRNA was studied here in experimentally introduced demyelination and remyelination by Cuprizone intoxication. During the demyelinating stage, both forms of MAG mRNA decreased markedly. When feeding with Cuprizone was stopped, MAG mRNA began to increase. One form of MAG mRNA without the exon 12 portion, which appears in normal development at the period of active myelination, was characteristically induced during the remyelinating stage. The other form containing the exon 12 portion was also induced but recovered only to the level in normal development.