Immunocytochemical studies have been carried out to determine the regional and cellular distribution of DARPP-32, a protein the phosphorylation of which can be regulated by dopamine and cAMP in intact cells. These immunocytochemical studies indicate tha DARPP-32 is localized primarily in those brain regions enriched in dopaminergic nerve terminals. Moreover, the staining pattern supports the conclusion that the DARPP-32 is present in dopaminoceptive neurons, i.e., neurons that receive a dopamine input, and that it is absent from the dopaminergic neurons themselves. Within the caudatoputamen, nucleus accumbens, olfactory tubercle, bed nucleus of the stria terminalis, and portions of the amygdaloid complex, all of which receive a strong dopamine input, DARPP-32 immunoreactivity is present in neuronal cell bodies and dendrites. In brain regions that are known to receive projections from these nuclei, puncta (presumed nerve terminals) are strongly immunoreactive for DARPP-32 but indigenous cell bodies and dendrites are not immunoreactive. These target areas include the globus pallidus, ventral pallidum, entopeduncular nucleus, and the pars reticulata of the substantia nigra. No immunoreactivity is detected in neuronal cell bodies or dendrites in any of the dopaminergic nuclei. Furthermore, nerve terminals immunoreactive for DARPP-32 do not resemble dopaminergic varicosities in either their morphology or their pattern of distribution. Many neurons are weakly immunoreactive for DARPP-32 and some of these are found in areas that apparently lack a dopaminergic input: weakly labeled neuronal cell bodies and dendrites were found throughout the neocortex, primarily in layer VI, and in the Purkinje neurons of the cerebellum. DARPP-32 immunoreactivity is also present in certain glial cells, especially in the median eminence, arcuate nucleus, and medial habenula. The present immunocytochemical studies, taken together with biochemical studies (Hemmings, H.C., Jr., A.C. Nairn, D.W. Aswad, and P. Greengard (1984) J. Neurosci. 4: 99–110; Walaas, S.I., and P. Greengard (1984) J. Neurosci. 4: 84–98) on DARPP- 32, indicate that DARPP-32, is present in the subclass of dopaminoceptive neurons containing D-1 receptors (dopamine receptors coupled to adenylate cyclase). DARPP-32 may be an effective marker for certain of the actions of dopamine that are mediated through cAMP and its associated protein kinase.