RNA was extracted from five different rat brain regions during development, starting from embryonic day 15 (E15) until postnatal day 60 (P60). These RNA preparations were analyzed by both Northern and dot blot for their content of 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase), myelin proteolipid protein (PLP), and myelin basic protein (MBP) -specific transcripts. CNPase mRNA was readily detectable at E15 and PLP mRNA at P1 in all brain regions examined. In contrast, expression of MBP mRNA followed a caudorostral gradient. It was first observed at P1 in the mesencephalon and at P9-P11 in the olfactory bulb. Expression of these three transcripts displayed two types of developmental profiles. One was termed biphasic because the specific mRNA level increased regularly and then reached a plateau level. The other developmental profile was termed triphasic, because there was a gradual increase in the level of specific transcripts with a sudden appearance of a sharp peak followed by a decline to a plateau level. When the triphasic pattern was observed, the date of the peak appearance was probe-, but not region-, dependent. It was P15 for CNPase, P18 for MBP, and P21 for PLP. As these peaks occurred at a time during development when myelination was the most active, we postulate the existence of a transient external signal, perhaps neuronal, which would be responsible for this increased amount of myelin-related transcripts.