Immunohistochemical double-labeling techniques were used to examine the co-localization of the neurotransmitter gamma-aminobutyric acid (GABA), the calcium-binding protein parvalbumin and the neurotensin-related hexapeptide LANT6 in neurons of the striatum and its target areas in pigeons and monkeys. The studies revealed the existence of a population of striatal interneurons apparently containing all three of these substances in both monkeys and pigeons. The results also revealed that GABA and LANT6 were co-localized in numerous pallidal and nigral reticulata neurons that also contained parvalbumin in both species. Examination of diverse other cell groups in avian forebrain and midbrain revealed that parvalbumin and LANT6 were typically co-localized to GABAergic neurons. In light of the presence of pallidal, reticulata and striatal neurons containing these three substances in two widely divergent amniote groups such as pigeons and monkeys, it seems likely that: (1) comparable neuronal populations are present in other avian and mammalian species; and (2) these neuronal populations play a fundamental role in basal ganglia functions that requires these three substances.