p38 Mitogen-activated protein kinase (p38 MAPK) is expressed in the oligodendrocyte lineage, and its activity has been implicated in the proliferation and transition of early progenitors into late progenitors. Although p38 MAPK expression has been found in the myelin sheath, however, its role in mature oligodendrocytes remains unknown. In the present study, in order to address the role of p38 MAPK in mature oligodendrocytes, selective inhibitors of p38 MAPK, SB202190, and SB203580 were added to primary cultures of mature oligodendrocytes. After 24h of exposure to the inhibitors, the appearance, and number of A2B5-positive progenitors were unchanged. However, the 2',3'-cyclic nucleotide-3'-phosphohydrolase-positive mature oligodendrocytes disappeared, and the numbers of living cells decreased in comparison to the control cells treated with SB202474, a negative analog of SB203580. Increases in the number of terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling-positive nuclei and in the activity of caspase-3/7 were detected 16 h after exposure to the inhibitors, thus causing the mature oligodendrocytes to die due to apoptosis. Similar results were obtained using a differentiated rat oligodendrocyte precursor cell (OPC) line, central glia-4 (CG-4). These findings indicate that p38 MAPK is vital for mature oligodendrocyte survival.