Somatostatin exerts its actions by means of a family of G protein-coupled receptors, five of which have so far been cloned. Whereas mRNAs for receptor subtypes sst 1 -sst 4 have been unequivocally localized in the brain, the data concerning the fifth subtype, sst 5 , are contradictory. Moreover, whereas sst 1 and sst 2A receptor proteins have been localized by immunohistochemistry, the distribution of sst 3 -sst 5 receptor proteins and/or subtype-specific binding remains to be determined in the central nervous system. In the present study, we investigated the distribution of immunoreactive sst 5 in adult rat brain and pituitary and demonstrated the presence of this receptor protein in the central nervous system by using an affinity-purified antibody generated against the C-terminus of the receptor. The specificity of the antibody for sst 5 was established by immunoblotting experiments on membranes prepared from cells transfected with cDNA encoding different somatotropin release inhibiting (SRIF) receptor subtypes as well as from anterior pituitary. In both systems, the antibody specifically recognized a band at approximately 50 kDa molecular mass, corresponding well to the reported size of the cloned receptor (48 kDa). Immunofluorescence in COS-7 cells transfected with individual SRIF receptor subtypes as well as in sections of rat pituitary demonstrated the antibody's applicability to the immunohistochemical detection of sst 5 receptors. In rat brain sections, sst 5 immunoreactivity was predominantly associated with neuronal perikarya and primary dendrites. Immunolabeling was most prominent in the olfactory tubercle, islands of Calleja, diagonal band of Broca, substantia innominata, and magnocellular preoptic nucleus of the basal forebrain as well as in the reticular nucleus of the thalamus. Other, less intensely labeled areas included the cerebral cortex, hippocampus, amygdala, preoptic area as well as the lateroanterior nucleus of the hypothalamus. The present findings provide the first characterization of the anatomic distribution of sst 5 receptors in the rat brain. They demonstrate a prominent expression of these receptors in the basal forebrain, suggesting that they may be involved in the mediation of somatostatin effects on the sleep-wake cycle through their association with cortically projecting subcortical systems.