Oligodendrocytes are responsible for myelination of axons within the central nervous system and are lost in the lesions of multiple sclerosis. The question of whether mature oligodendrocytes are capable of division, and hence of giving rise to new myelinating cells, remains controversial. We have studied the response in vitro of rat oligodendrocytes to growth factors. Unlike their progenitors which divided vigorously in response to growth factor stimulation, mature oligodendrocytes showed no mitotic response to a number of individual growth factors. However, when exposed to basic fibroblast growth factor (bFGF), mature oligodendrocytes expressed the immediate response gene c-fos and the cell cycle-associated protein proliferating cell nuclear antigen, but failed to divide and ultimately underwent cell death by apoptosis. These findings are consistent with the emerging view that mitosis and apoptosis are closely linked phenomena.