Protease M/neurosin is a serine protease expressed by oligodendrocytes (OLGs) in the central nervous system (CNS). To investigate the role of protease M/neurosin during experimental demyelination and remyelination, mice were fed cuprizone (bis-cyclohexanon oxaldihydrazone). Semi-quantitative RT-PCR analysis and immunohistochemistry revealed that the expressions of protease M/neurosin mRNA and protein were rapidly reduced in demyelination, whereas the expression of protease M/neurosin was increased in pi form of glutathione-S-transferases (GST-pi)-positive OLGs during remyelination. Cultured primary OLGs displayed a strong correlation between protease M/neurosin and myelin basic protein (MBP). After tumor necrosis factor-alpha (TNF-alpha) and IFN-gamma stimulation, these proteins showed colocalization in the oligodendroglial process. The suppression of protease M/neurosin using RNAi reduced the level of MBP mRNA in cultured OLGs. In contrast, the reduced level of protease M/neurosin was not associated with oligodendroglial cell death or differentiation in cultured OLGs. This study identifies that protease M/neurosin in OLGs is closely associated with the expression of the MBP and the PLP gene. Our data emphasize that the maintenance of myelination is an important function of protease M/neurosin in OLGs, suggesting its relation to the oligodendroglial response to myelin disorders.