The major rat brain microtubule-associated proteins, MAP2 and tau, exhibit various properties that implicate them in the mechanisms underlying the growth of axons and dendrites during neuronal development. To determine if these properties represent fundamental morphogenetic mechanisms, we have examined the phylogenetic conservation of these proteins in Xenopus laevis, quail and rat with respect to their molecular form, cytological distribution and developmental expression. In all three species, the high-molecular weight form of MAP2 migrates as a pair of polypeptides (MAP2a and MAP2b); this doublet as well as the low-molecular weight form of MAP2 (MAP2c) and the tau proteins are markedly similar in size in the different classes of vertebrates. Immunohistochemical staining of the Xenopus and quail cerebellum showed that MAP2 is highly concentrated in dendrites whereas the tau proteins are predominantly confined to axons, exactly as they are in rat. The developmental regulation of these proteins in Xenopus and rat is also conserved. Between the larva and the adult (i.e. during metamorphosis) MAP2c undergoes a marked decrease while MAP2a undergoes a large increase. Thus, in both classes of vertebrates the timing of changes in MAP2 expression coincides with the maturation of neuronal morphology. Taken together, these conserved properties of MAP2 and tau in three phylogenetically divergent classes of vertebrates suggest that these proteins serve fundamental functions during neuronal morphogenesis.