The concentrations of both somatostatin-like immunoreactivity (SLI) and neuropeptide Y-like immunoreactivity (NPYLI) were measured in all major cortical and subcortical regions of monkey brain. High concentrations of both SLI and NPYLI were found in cerebral cortex where they were significantly correlated, supporting a colocalization of the two peptides in cortical neurons. There were no significant differences between the right and left hemispheres. Subdissections of the basal ganglia showed that concentrations of both SLI and NPYLI were 3-fold higher in the nucleus accumbens than in the remainder of the striatum, where concentrations of the peptides were evenly distributed. SLI and NPYLI levels were significantly correlated in the striatum, supporting a colocalization of the two neuropeptides in striatal neurons. Concentrations were low in the globus pallidus. Limbic system nuclei including the amygdala, bed nucleus of the stria terminalis and substantia innominata had relatively high concentrations of both SLI and NPYLI. In several subcortical regions concentrations of the neuropeptides were dissociated, suggesting separate neuronal populations. The distribution of both neuropeptides in monkey brain closely resembles that in human brain. The high levels in cerebral cortex and basal ganglia suggest that somatostatin and neuropeptide Y may play a role both in cognition and in normal motor function.