Myelination in the CNS is accompanied by the differentiation of oligodendrocytes as well as the coordinate expression of a group of myelin-specific genes, including those encoding proteolipid protein and myelin basic protein. In order to compare the timing of the onset of myelin gene expression with the known sequence of oligodendrocyte maturation, we analyzed cerebral white matter cultures grown in the presence of platelet-derived growth factor for expression of the mRNAs encoding these myelin proteins, as well as for the numbers of oligodendrocytes and their precursors. Platelet-derived growth factor treatment increased the rate of oligodendrocyte precursor cell proliferation and the number of mature oligodendrocytes. Platelet-derived growth factor also produced a significant increase in oligodendrocyte precursors prior to an increase in their proliferation rate, suggesting that platelet-derived growth factor may also have an effect on oligodendrocyte precursor survival. Furthermore, steady-state levels of proteolipid protein and myelin basic protein mRNAs increased within 24 of the addition of platelet-derived growth factor, before any significant change in the numbers of oligodendrocytes or their precursors, demonstrating that platelet-derived growth factor also regulates myelin gene expression. At later times after platelet-derived growth factor addition, however, when the number of oligodendrocytes and their precursors was rapidly increasing, the increase in proteolipid protein and myelin basic protein mRNA levels was proportionally much greater than the increase in oligodendroglial lineage cells, suggesting that platelet-derived growth factor also increased the number of proteolipid protein and myelin basic protein transcripts per cell; this interpretation was confirmed by in situ hybridization analysis. Finally, by examining the co-expression of galactocerebroside using the epitopes recognized by the Ranscht monoclonal antibody and proteolipid protein mRNA in individual cells by a combination of in situ hybridization and immunohistochemistry, we demonstrated that oligodendrocytes express proteolipid protein and myelin basic protein mRNA. Oligodendrocyte maturation, as measured by surface galactocerebroside expression, is thus contemporaneous with the activation of myelin-specific gene expression.