Mammalian microtubule-associated protein 2 (MAP2) exists in high-molecular-weight (Mr approximately 280,000) and low-molecular-weight (Mr approximately 70,000) forms, with the latter protein being more abundant in embryonic brain homogenates than in preparations from mature brain (Riederer and Matus, 1985). In the current study, we have shown that avian MAP2 also exists as both high- (Mr approximately 260,000) and low-molecular-weight (Mr approximately 65,000) forms whose relative abundance changes during brain maturation, indicating a conserved function for these proteins during vertebrate neuronal morphogenesis. Using indirect immunohistochemistry, we have determined the cellular distribution of the high- and low-molecular-weight forms of MAP2 in the developing avian cerebellum. In the embryonic cerebellum, low-molecular-weight MAP2 is found in the external granular layer and in epithelial cells. High-molecular-weight MAP2 is found only in neurons that have commenced dendrogenesis, i.e., Purkinje cells and neurons within the internal granular layer. Thus, low-molecular-weight MAP2 is not only more abundant in embryonic nervous tissue than in the adult, but it also appears in glia and in differentiating neurons before the high-molecular-weight form. We have also shown that in the mature cerebellum high-molecular-weight MAP2 cannot be detected with monoclonal antibodies or polyclonal antisera in Purkinje cell dendrites. Polyclonal antisera against the regulatory subunit of the cAMP-dependent protein kinase, which is associated with MAP2 in the Purkinje cell dendrites of the rat, also fail to stain Purkinje cell dendrites in the mature quail cerebellum. This suggests that high-molecular-weight MAP2 may be necessary for the establishment of dendrites but is not necessary for the maintenance of dendritic form.