A detailed comparison in the developing rat central nervous system between the distribution of the NG2 proteoglycan and the alpha-receptor for platelet-derived growth factor (PDGF) shows that these two molecules are co-expressed by glial progenitor cells of the O2A lineage and can serve as reliable markers for identification of O2A cells in vivo. Our mapping experiments indicate that NG2-positive, PDGF alpha-receptor positive O2A cells are abundant throughout the developing central nervous system in both white and gray matter. The earliest cells immunoreactive for either of the two markers are found adjacent to the central canal of the embryonic day 15 (E15) spinal cord. These cells express only PDGF alpha-receptor and not NG2. By E17, process-bearing cells expressing both NG2 and PDGF alpha-receptor in a highly co-localized fashion are found throughout the central nervous system. The first postnatal week marks the peak in the number of NG2 and PDGF alpha-receptor immunoreactive cells, as well as the peak in the level of expression and the extent of co-localization of the two molecules. After the first week, the level of expression of both NG2 and PDGF alpha-receptor declines, although both molecules continue to be expressed in the adult brain. On O2A cells in the mature brain, NG2 and PDGF alpha-receptor are not as well co-localized at the subcellular level as they are on O2A cells in the younger brain. The functional consequences of co-localization and subsequent dissociation of NG2 and PDGF alpha-receptor on maturing O2A progenitors are investigated in the accompanying paper (Nishiyama et al.: J Neurosci Res 43:315-330, 1996).