This article describes the isolation of a microtubule-associated protein 1B (MAP1B) cDNA clone from a rat brain lambda gt11 library and the study of MAP1B mRNA expression during brain development. On Northern blots, the cDNA hybridized with an mRNA of greater than 10 kilobases which was present only in the brain. The identity of the cDNA was confirmed by the characterization of the antiserum against the fusion protein, and also by comparing both the original antibody and the anti-fusion protein antiserum with a panel of well-studied monoclonal antibodies against different forms of MAP1 and MAP2. The regulation of MAP1B mRNA during development was studied in whole brain, cerebral cortex, hypothalamus, brainstem, and olfactory bulbs. The steady-state levels of MAP1B mRNA in all tissues examined were relatively low in the adult compared to developing brains. This decrease varied in different brain regions, and its time course appeared to coincide with the pattern of postnatal developmental and morphological events. The developmental patterns of the MAP1B mRNA and protein in the brain were similar, suggesting that expression of this protein is under transcriptional control. The RNA blots were also probed with beta-actin and beta-tubulin to compare the levels of MAP1B mRNA with other cytoskeletal elements and as controls for the quality of the RNA.