Developmental regulation in mRNAs of three brain proteins has been investigated by Northern blot evaluation in C57BL/6 mice. The mRNAs of two cytoskeletal components, glial fibrillary acidic protein (GFAP) and beta actin, varied significantly, and differently, during brain development (0-56 days postnatal). The beta actin mRNAs peaked at day 1 after a slight increase, then dropped rapidly during the first 15 days postnatal, and thereafter remained at a level which was strictly maintained throughout development and adulthood. Conversely, the GFAP mRNAs increased during the first two weeks after birth (astroglial proliferation), and then slightly declined until the adult stage (astroglial cell differentiation). The prion protein (PrP) mRNAs were detectable as soon as birth, and increased 4-fold during brain maturation. Then, during the adult life, the GFAP and PrP mRNAs did not change markedly. Nevertheless, slight but significant increases in the mRNA levels of both GFAP and PrP were observed at older stages (360 days). These results are analysed in the light of the implications of PrP and GFAP in scrapie infection models.