Expression of mRNAs for the two major myelin proteins, myelin basic protein (MBP) and proteolipid protein (PLP), was examined in a number of regions of the developing mouse brain using in situ hybridization. In general, MBP and PLP mRNAs were observed to be coexpressed during ontogeny, prior to the histological appearance of myelin. Expression of both mRNAs was detected as early as 6 hours postpartum in the medulla oblongata and, with development, expression of these mRNAs progressed in a caudal to rostral direction. Peak expression occurred at approximately postnatal day 20 in most regions examined, regardless of time of onset of expression. As myelination proceeded, two different labeling patterns were observed with the PLP and MBP 35S-labeled cDNA probes. In the earliest stages of myelinogenesis MBP mRNA labeling was restricted to oligodendrocyte cell bodies, but shortly after the gene began to be expressed the labeling became more diffuse. In contrast, PLP mRNA labeling remained over or surrounding oligodendrocyte cell bodies at all stages of myelinogenesis. These two distinctly different patterns of labeling are consistent with alternative intracellular trafficking of MBP and PLP mRNAs, in which PLP mRNAs remain associated with ribosomes within the cell soma and MBP mRNAs move from the cell soma to the oligodendrocyte processes at a specific stage early in myelinogenesis. However, there appeared to be a clear time lag between the onset of MBP mRNA expression and the movement of ribosomes carrying MBP mRNAs into the oligodendrocyte processes. Additionally, the in situ hybridization studies revealed a population of unidentified cells residing in cortical molecular layers that express PLP mRNA in the absence of MBP mRNA.