Although oligodendrocytes are vulnerable to focal cerebral ischemia, remyelination of denuded or regenerating axons in the peri-infarct area has been observed in the central nervous system. We studied the expression of myelin basic protein (MBP), a major component of central nervous system myelin, in peri-infarct areas in adult rat brain after transient middle cerebral artery occlusion (MCAO) and correlated it to the expression of the growth-associated protein-43 (GAP-43), a marker for axonal regeneration and sprouting, using nonradioactive in situ hybridization techniques. Within the infarct, MBP messenger RNA (mRNA) had disappeared by 24 h, whereas myelin protein, identified by MBP and myelin oligodendrocyte glycoprotein (MOG) immunohistochemistry, appeared structurally intact until day 3. Peri-infarct oligodendrocytes increased their expression of MBP mRNA from 24 h to maximal levels at day 7, corresponding to the appearance of process-bearing MBP and occasional MOG-immunoreactive oligodendrocytes in parallel sections. Quantitative analysis revealed significant increases in the density of oligodendrocytes (up to 7.6-fold) and in the level of MBP mRNA expressed by individual cells. Parallel sections showed that increased expression of GAP-43 mRNA in neurons was concomitant to MBP mRNA upregulation in oligodendrocytes. While the mechanisms regulating oligodendrocyte survival and myelination signals are not clear at this point, axonal sprouting could putatively serve as a stimulus for the upregulation of oligodendrocyte cell numbers, differentiation state, and/or active myelination in the peri-infarct areas.