Neuropeptide Y and somatostatin immunoreactive neurons and processes were examined in human striatum using both immunofluorescence and avidin biotin immunoperoxidase methods. Reduced nicotinamide adenine dinucleotide phosphate diaphorase activity was histochemically determined by the reduction of nitro blue tetrazolium. Immunofluorescence using a monoclonal anti-somatostatin antibody and a polyclonal anti-neuropeptide Y antibody, followed by diaphorase histochemistry, showed that these three neurochemical markers are co-localized in a single population of medium-sized aspiny intrinsic neurons. Cells were evenly distributed in clusters throughout the striatum, but fiber density was higher in the nucleus accumbens and ventromedial regions of the caudate and putamen. Double-stained reduced nicotinamide adenine dinucleotide phosphate diaphorase-acetylcholinesterase sections demonstrated that these neurons are located in zones of high acetylcholinesterase activity, often at the interface of these zones with regions of low enzyme activity. These biochemically distinctive neurons are uniquely situated to modulate activity between striatal compartments. Our findings provide new information about the modular organization of the striatum and extend these observations in human brain.