In order to map the recently cloned serotonin 5-HT6 receptor in the rat brain and spinal cord, polyclonal antibodies were raised against a synthetic octadecapeptide corresponding to a specific portion (Leu398-Val415) of the C-terminal domain of this receptor. Antibodies were detected by enzyme-linked immunosorbent assay as soon as one month after the first injection to rabbits of the peptide coupled to keyhole limpet hemocyanin. Immunoautoradiographic experiments with antibodies affinity-purified on Affi-Gel coupled to the peptide antigen showed that 5-HT6-like immunoreactive material was abundant in the olfactory tubercle (plexiform layer), cerebral cortex (frontal and entorhinal areas), nucleus accumbens, striatum, hippocampus (strata oriens and radiatum of the CA1 area, molecular layer of the dentate gyrus) and the molecular layer of the cerebellum. A specific immunolabeling, but at moderate intensity, was also observed in the thalamus, substantia nigra, superficial layer of the superior colliculus, motor trigeminal nucleus and facial nucleus. In contrast, no 5-HT6-like immunoreactive material was found in white matter areas. As the regional distribution of 5-HT6 receptor-like immunoreactivity matched generally that previously found for the 5-HT6 receptor mRNA, one could infer that this receptor protein is addressed in the vicinity of its synthesis site, i.e. on somas and/or dendrites. Indeed, immunohistochemistry at the light and electron microscope level showed that 5-HT6-like immunoreactivity was associated with dendritic processes in both the striatum and the dentate gyrus of the hippocampus. The relative abundance of 5-HT6 receptor-like immunoreactivity in extrapyramidal and limbic areas suggests that 5-HT6 receptors may participate in the serotoninergic control of motor function and mood-dependent behavior, respectively.