We have examined the ontogeny of the microtubule-associated protein, MAP2 in rat cerebellum using biochemical and immunocytochemical techniques. In adult animals MAP2 can be resolved as a doublet of polypeptides (referred to here as MAP2a and MAP2b). In young rats only the lower molecular weight MAP2b was present in cerebellar cytosol; MAP2a appeared by 20 days postnatally. The identification of the polypeptides as MAP2a and MAP2b in heat-stable fractions of cytosol was confirmed by peptide mapping which also demonstrated that MAP2a and MAP2b shared virtually identical peptide maps. Using a rabbit anti-MAP2 antibody that was characterised by immunoblotting of cerebellar homogenates, the localisation of MAP2 was examined in 10-day-old and adult cerebella. In 10-day-old animals, MAP2 immunoreactivity was detected in the external germinal layer, granular layer and most strikingly in Purkinje cell bodies and dendrites. Parallel fibre axons were unstained. In contrast both axons and dendrites were stained by an antibody against alpha-tubulin. In adult animals MAP2 immunoreactivity was largely confined to Purkinje cell dendrites. The absence of assembled microtubules from Purkinje cell dendrites of young animals despite the presence of MAP2 and tubulin immunoreactivity may be explained by the presence of only a single MAP2 polypeptide (MAP2b) whereas the presence of the MAP2 doublet polypeptides in the adult may facilitate polymerization. Since MAP2b does not co-assemble with microtubules efficiently in vitro it may represent a form of MAP2 unable to stimulate microtubule assembly. Therefore, the possibility exists that MAP2b may play an alternative role during dendritic growth.