Oligodendrocyte precursors (OLPs or 'NG2 cells') are abundant in the adult mouse brain, where they continue to proliferate and generate new myelinating oligodendrocytes. By cumulative BrdU labelling, we estimated the cell cycle time TC and the proportion of NG2 cells that is actively cycling (the growth fraction) at approximately postnatal day 6 (P6), P60, P240 and P540. In the corpus callosum, TC increased from <2 days at P6 to approximately 9 days at P60 to approximately 70 days at P240 and P540. In the cortex, TC increased from approximately 2 days to >150 days over the same period. The growth fraction remained relatively invariant at approximately 50% in both cortex and corpus callosum - that is, similar numbers of mitotically active and inactive NG2 cells co-exist at all ages. Our data imply that a stable population of quiescent NG2 cells appears before the end of the first postnatal week and persists throughout life. The mitotically active population acts as a source of new oligodendrocytes during adulthood, while the biological significance of the quiescent population remains to be determined. We found that the mitotic status of adult NG2 cells is unrelated to their developmental site of origin in the ventral or dorsal telencephalon. We also report that new oligodendrocytes continue to be formed at a slow rate from NG2 cells even after P240 (8 months of age).