Proteolipid protein (PLP) and DM20, two abundant proteins of myelin, are produced from alternatively spliced mRNAs from the primary PLP gene transcript. Recent studies on the mouse, bovine and human central nervous systems have found that DM20 protein is expressed prior to PLP during development. As development proceeds, however, PLP becomes the predominant protein. This implies that there must be some form of regulation controlling the ratio of these proteins during development. If this regulation occurs during transcription or splicing, the PLP/DM20 mRNA ratio should mimic the PLP/DM20 protein ratio during development. In order to study these closely related mRNAs, we developed a method that used oligonucleotide probes to specifically identify PLP or DM20 mRNA. In this study, we established that (1) as with other species studied, the DM20 protein is present at higher or equivalent levels to PLP during early rat brain development, and (2) PLP and DM20 mRNAs have essentially identical developmental profiles in the rat, although the DM20 mRNA is expressed at lower levels than PLP mRNA. Since the PLP/DM20 protein ratio in young rats does not reflect the PLP/DM20 mRNA ratio, the mechanism of regulation responsible for altering the PLP/DM20 protein ratio during development must occur after transcription and splicing. Possible posttranscriptional mechanisms controlling the ratio of these proteins during development are discussed.