In developing brain, Eph receptors and their ephrin ligands (Ephs/ephrins) are implicated in facilitating topographic guidance of a number of pathways, including the nigrostriatal and mesolimbic dopamine (DA) pathways. In adult rodent brain, these molecules are implicated in neuronal plasticity associated with learning and memory. Cocaine significantly alters the expression of select members of this family of axonal guidance molecules, implicating Ephs, ephrins in drug-induced neuroadaptation. The potential contribution of Ephs, ephrins to cocaine-induced reorganization of striatal circuitry brain in primates [Saka, E., Goodrich, C., Harlan, P., Madras, B.K., Graybiel, A.M., 2004. Repetitive behaviors in monkeys are linked to specific striatal activation patterns. J. Neurosci. 24, 7557-7565] is unknown because there are no documented reports of Eph/ephrin expression or function in adult primate brain. We now report that brains of adult old and new world monkeys express mRNA encoding EphA4 receptor and ephrin-B2 ligand, implicated in topographic guidance of dopamine and striatal neurons during development. Their encoded proteins distributed highly selectively in regions of adult monkey brain. EphA4 mRNA levels were prominent in the DA-rich caudate/putamen, nucleus accumbens and globus pallidus, as well as the medial and orbitofrontal cortices, hippocampus, amygdala, thalamus and cerebellum. Immunocytochemical localization of EphA4 protein revealed discrete expression in caudate/putamen, globus pallidus, substantia nigra, cerebellar Purkinje cells, pyramidal cells of frontal cortices (layers II, III and V) and the subgranular zone of the hippocampus. Evidence for EphA4 expression in dopamine neurons emerged from colocalization with tyrosine-hydroxylase-positive terminals in striatum and substantia nigra and ventral tegmental area cell bodies. The association of axonal guidance molecules with drug-induced reorganization of adult primate brain circuitry warrants investigation.