The levels of three different microtubule-associated proteins (MAP1, -2, and -3) in brain were found to undergo large changes during postnatal development. MAP1 was barely detectable at birth but thereafter steadily increased, reaching adult levels by postnatal day 20 (P20). Both MAP2 and MAP3 showed differential expression patterns of their component peptides. At birth, MAP2 was represented by the smaller of two Mr 280,000 peptides (MAP2b) and three antigenically related Mr 70,000 peptides. The larger of the Mr 280,000 peptides (MAP2a) first appeared between P10 and P20, and the Mr 70,000 components disappeared at the same time. Of the two MAP3 peptides, the larger (MAP3a) was present in the late embryo, several days before MAP3b appeared. Between P10 and P20, both MAP3 components underwent a striking decrease in abundance (a factor of 10), which correlated with their disappearance from all neuronal compartments except neurofilament-containing axons. These developmental changes in expression are different and characteristic for each of the three MAPs, yet in each case they are detectable in brain homogenates, indicating that they occur concurrently throughout the brain.