In vitro studies indicate that fibroblast growth factor 2 (FGF2) has diverse effects on cells of the early and late oligodendrocyte lineage. Here, we have examined this in vivo by comparing the actions of FGF2 on the developing and developed anterior medullary velum (AMV) of postnatal rats. FGF2, or saline vehicle in controls, was administered into the cerebrospinal fluid of anaesthetised rats between postnatal day (P)6 and P9 either for 1 day (1d), 2d, or 3d, and AMV were analysed at P8 or P9. Immunolabelling for NG2 was used to identify oligodendrocyte progenitor cells (OPCs) and Rip for premyelinating and myelin-forming oligodendrocytes. At P6-9, the AMV was clearly demarcated into myelinated caudal and premyelinated rostral areas. The caudal AMV was populated by differentiated myelin-forming oligodendrocytes and ‘adult’ OPCs, whilst the rostral AMV contained mixed populations of ‘perinatal’ OPCs, and both premyelinating and myelin-forming oligodendrocytes. Administration of FGF2 resulted in the accumulation of OPCs in both the developing and developed AMV. Notably, FGF2 had a bipartite action on premyelinating oligodendrocytes, at first dramatically expanding their population throughout the premyelinated and myelinated AMV, but subsequently causing the loss of these previously generated cells. In addition, FGF2 induced the loss of existing myelin-forming oligodendrocytes in the developed AMV, and arrested the generation of new myelin-forming cells in the developing AMV. This study provides evidence that FGF2 has opposing positive and negative actions on early and late oligodendrocyte lineage cells in vivo.