We report the localization of PDGFR alpha mRNA (PDGFR alpha) in phenotypically defined cells during the first postnatal week of rat forebrain development. Using a method of combined immunocytochemistry and in situ hybridization we have demonstrated the cellular colocalization of PDGFR alpha mRNA with GD3 ganglioside or O4 sulfatide, phenotypic markers of oligodendrocytes, in the gray and white matter of the dorsal cerebral cortex at all ages studied. Population analysis of the PDGFR alpha +/GD3+ and PDGFR alpha+/O4+ cells revealed that three populations express PDGFR alpha: GD3+, GD3+/O4+, and O4+, corresponding to two lineage stages, progenitor and preoligodendrocyte, in oligodendrocyte development. Immature oligodendrocytes, identified by galactocerebroside immunoreactivity, did not express detectable levels of PDGFR alpha mRNA. Post-mitotic neurons, identified by immunoperoxidase localization of the 68 kD neurofilament, and astrocytes identified by S-100 or GFAP immunoreactivity were also negative for PDGFR alpha mRNA. The spatial and temporal expression of PDGFR alpha mRNA occurred in oligodendrocyte cell populations which are post-migratory and proliferative, but which do not express myelin proteins characteristic of post-mitotic oligodendrocytes.