Dopamine and adenosine 3':5'-monophosphate (cAMP) regulated phosphoprotein of M(r) 32 kDa (DARPP-32) and phosphatase inhibitor 1 (I-1) have been associated with intracellular signal transduction processes and share several biochemical features. Localization of each phosphoprotein in distinct neural structures will aid investigation of their physiologic properties and help identify their unique roles in the nervous system. We have compared the distribution of the two phosphoproteins in the amygdala and hippocampus of the rhesus monkey with the aid of immunocytochemical procedures. Neurons immunoreactive to antibodies raised against the phosphoproteins DARPP-32 and I-1 were noted in the cortical, central, and components of the basal group, including the basomedial, the lateral, and to a lesser extent, the basolateral amygdaloid nuclei. Within the large basal nuclei positive neurons were found preferentially in their medial and ventral subdivisions. By making a direct comparison in the same animals, we observed differences in the distribution of the two phosphoproteins in the amygdala. DARPP-32 and I-1 positive neurons overlapped partially in the basal nuclei, to a lesser extent in the cortical, but were segregated in the central amygdaloid nucleus with neurons positive for DARPP-32 noted laterally, and for I-1 medially. In contrast to the amygdala, where numerous DARPP-32 and I-1 positive neurons were observed, only I-1 had a notable presence in the hippocampus. Moreover, I-1 associated label was found only in neurons in the granule cell layer of the dentate gyrus, their dendritic plexus, and axons which innervate hilar and CA3 neurons. DARPP-32 and I-1 are intracellular messengers associated with signal transduction. Their regional distribution in the amygdala and the hippocampus suggests an involvement in the level of excitability of specific components of these limbic structures. Moreover, our results suggest that I-1 has a unique role in the intrinsic circuitry of the hippocampal formation and indicate a system where the physiologic properties of I-1 may be studied in isolation.