1. In order to characterize some of the molecular events leading to repair of myelin in the adult central nervous system (CNS), we examined the expression of transcripts for myelin basic protein (MBP) during remyelination in the mouse. C57B1/6 mice develop a demyelinating disease when glial cells are selectively infected by the A59 strain of mouse coronavirus. The virus is spontaneously cleared from the mice by 4 weeks postinfection (WPI), a time when remyelination is starting. 2. At 3 WPI total MBP transcripts are decreased by 75% in demyelinating lesions compared to control white matter. Using RNase protection assays and in situ hybridization with probes for particular MBP exons, we detected an increase in MBP transcripts containing exon 2 information, coincident with the earliest histological signs of remyelination. 3. The expression of MBP transcripts containing exon 2 information was first seen clustered in the perinuclear cytoplasm of oligodendrocytes scattered within the lesions. This is reminiscent of the increased levels and perinuclear clustering of MBP transcripts containing exon 2 seen during early developmental myelination. The peak abundance of exon 2-containing transcripts in the lesions was 13-fold that seen in control white matter. At later stages of remyelination, additional forms of MBP transcripts (without exon 2) increased and their distribution was more diffuse. 4. Thus, during remyelination, preforms of MBP transcripts, which are normally present at low levels in the adult CNS, are abundantly expressed and regulated in a manner similar to that observed in developmental myelination.