ARPP-21, a cAMP-regulated phosphoprotein, has been studied by immunocytochemistry to determine its cellular and regional distribution in rat brain. This study demonstrates that ARPP-21 immunoreactivity is present throughout the cytoplasm of immunoreactive neurons and that most of the immunoreactivity is associated with the basal ganglia. Within the caudatoputamen (CP), nucleus accumbens, olfactory tubercle, bed nucleus of the stria terminalis, and portions of the amygdaloid complex, ARPP-21 is present in neuronal somata and dendrites. In brain regions known to receive projections from these nuclei, immunoreactivity is present in puncta (presumed axons and axon terminals). These regions include the globus pallidus, ventral pallidum, entopeduncular nucleus, lateral preoptic area, and substantia nigra. Within the basal ganglia, ARPP-21 immunoreactivity is most intense in the olfactory tubercle, nucleus accumbens, medial portion of the CP, and the ventral retrochiasmatic pocket of the CP. These same areas comprise the limbic striatum, and ARPP-21 is the first substance found to be specifically enriched therein. The possibility is discussed that ARPP-21 mediates effects of multiple first messengers, including dopamine and vasoactive intestinal polypeptide, that act through cAMP.