The expression of myelin proteins in actively myelinating quaking and control brains was studied. RNA was extracted from the brains of 18- and 27-day-old mice and analyzed by northern blot using cDNA probes for proteolipid protein (PLP), basic protein (BP), and myelin-associated glycoprotein (MAG). Two PLP transcripts of 3.2 and 2.4 kb (kilobase) were found, whereas PB and MAG probes hybridized to single regions of 2.2 and 2.5 kb, respectively. No abnormality in the transcript pattern was detectable in the quaking brain at either 18 or 27 days of age. Over this 9-day period the level of PLP and BP message in the control brain decreased by approximately 10%, whereas the level of MAG message decreased by approximately 50%. In the 18-day-old quaking brain the expression of PLP and BP was severely reduced amounting to one-third and one-half of the control values, respectively. The reduction at the age of 27 days was less. On the other hand, the quaking brain produced more MAG mRNA amounting to 1.6- and 3.2-fold control on the 18th and 27th day. The results indicate a reduced expression of the PLP and BP genes and a developmental delay in the mutant, whereas the genetic expression of MAG is enhanced and appears to be progressively dysregulated.