A specific antiserum was used to ascertain the distribution of microtubule-associated protein 2 (MAP2) in the rat brain at the light and electron microscope levels. Light microscopy showed MAP2 to be present only in neurons, and only in the dendrites and the perikaryon of each cell. This same polarized distribution pattern was found in the Purkinje, Golgi, basket, stellate, and granule cells of the cerebellum, and also in neurons of the hippocampus, the olfactory bulb, and the midbrain. While labelling of the dendritic arborization was extensive and intense, MAP2 density tended to decrease in the proximal dendritic trunk. Particularly in large neurons (e.g., Purkinje, Golgi, and pyramidal cells), staining was reproducibly weaker in the cell body than in the main dendrites. Dendritic contours generally appeared smooth, without any evidence of staining of dendritic spines. An electron microscope examination of the cerebellum confirmed the presence of MAP2 reactivity in neurons and its absence from axons and non-neuronal cells. MAP2 in dendrites was associated with microtubules, while MAP2 in neuronal perikarya was associated with polyribosomes. There was no evidence of specific staining in dendritic spines and in postsynaptic densities. MAP2 is a novel dendritic marker and labels part of a specific dendritic cytoskeleton, different from that in axons and non-neuronal cells.