In the present paper immunocytochemical analysis at the fluorescence microscopical level has been performed of neural cell adhesion molecule (NCAM) immunoreactivity in the adult rat tel- and diencephalon in order to further substantiate the highly selective neuronal localization of NCAM immunoreactivity, using an affinity purified rabbit antiserum recognizing homologous NCAM proteins from rat brain. Also, double immunolabelling experiments were performed with monoclonal antibodies specific for heparan sulfate related epitopes or gamma-aminobutyric acid (GABA) to establish in which cell populations a colocalization existed with immunoreactive heparan sulfate proteoglycans or GABA. Within the neocortex NCAM immunoreactivity was exclusively localized to the area of the cell membrane of soma and proximal dendrites of subsets of large pyramidal nerve cells of the layer 5 of the frontoparietal cortex. Within the dorsal hippocampus, the NCAM immunoreactivity was exclusively located to the cell surface area of the pyramidal cell bodies of area CA2. Two colour immunofluorescence procedures demonstrated a colocalization of NCAM and 3G10 but not 10E4 immunoreactivities in the cell surface area of many of the NCAM-positive nerve cell bodies of these two regions. Within the thalamus, strong NCAM immunoreactivity was exclusively demonstrated at all rostrocaudal levels of the reticular thalamic nucleus. The horizontal band of NCAM immunoreactivity was not continuous, but split up into patches of NCAM immunoreactivity within groups of nerve cell bodies. When analysing the number of cells per unitary square in the rostrocaudal direction, a significant increase of positive cells was found in the rostral and middle thirds versus the caudal third of the reticular thalamic nucleus. Many of the cell bodies with NCAM immunoreactivity in their cell surface area showed cytoplasmic GABA immunoreactivity. In the three regions shown to contain NCAM immunoreactivity, proteins of the NCAM type may play a special role for the maintenance of the synaptic structure. The findings also suggest that the sulfated proteoglycans and NCAM can interact in the regulation of cell-cell interaction via adhesion. In the reticular thalamic nucleus NCAM molecules may be part of a set of cell-adhesion molecules involved in a structural organization of the nucleus, which allows it to play a key role in relating cortical maps to thalamic maps.