The rat olfactory tubercle was studied with the rapid Golgi method. Several distinct cell types were identified mainly on the basis of the size of their somata and the structure of their dendrites. The commonest neuron type in the tubercle is the medium-sized densely spined cell. The somata of these neurons occur chiefly in the dense cell and multiform layers. They also form the cell bridges that directly link the olfactory tubercle with the nucleus accumbens and caudate-putamen. Their dendritic trees exhibit a variety of shapes; some of them are spherical, some are bipolar, and others are asymmetrical. The axons project dorsally, deep into the multiform layer. En route they give off numerous collaterals. A large version of this cell type is the crescent cell. Other medium-sized neurons also have somata in the dense cell and multiform layers. They include the spindle cells, so named because of the shape of their cell bodies, and the medium-sized spine-poor neurons. Neither of these cell types has dendritic trees that are as highly branched as those of the medium-sized densely spined cells. There are three types of small cells; their somata occur primarily in the dense cell and molecular layers. The dwarf cells are near the pial surface, although their somata are included in the dense cell layer, and they have axons that resemble those of medium-sized densely spined cells. The radiate cells have numerous, relatively short, spine-free dendrites that extend out from the rounded somata in all directions. The small spine-rich cells look like miniature versions of the medium-sized densely spined neurons. They are frequently confined to the molecular layer. Large spine-poor neurons, with their cell bodies located in the dense cell and multiform layers, seem to be a heterogeneous cell group since there are subtle variations in the structure of their dendrites and the shape and extent of their dendritic trees. The large, moderately spined neurons are less common than the other large cells; their somata are found in all three layers. The granule cells of the islands of Calleja make up the most homogeneous cell group. They have only a few dendrites, and these are quite thin. Except for the medium-sized densely spined and dwarf cells, the axons of the different cell types were not very well impregnated. The different cell types in the tubercle are compared to cells in the nucleus accumbens, caudate-putamen, and globus pallidus.(ABSTRACT TRUNCATED AT 400 WORDS)