Endogenous monoamine concentrations and turnover rates vary markedly in different regions of neocortex as well as in various subcortical structures of young adult rhesus monkeys. Monoamine levels and synthesis rates in amygdala, hippocampus, neostriatum, thalamus and brain stem are generally similar to comparable measures previously reported in a variety of species. However, extending and confirming the results of an earlier study, cortical monoamines exhibit topographically specific patterns of distribution. Thus, dopamine concentration is highest in the prefrontal and temporal neocortex; it decreases along the fronto-occipital axis and only trace amounts are detectable in the visual cortex. The distribution of norepinephrine is similar to that of dopamine except that the highest concentrations of norepinephrine are found in somatosensory cortex instead of prefrontal cortex. The pattern of distribution of serotonin is more uniform. However, the distribution of its metabolite, 5-hydroxyindoleacetic acid, is complementary to that of dopamine: the concentration is lowest in prefrontal cortex and highest in posterior regions of the telencephalon. Synthesis of catecholamines as measured by DOPA accumulation in monkeys treated with an aromatic amino acid decarboxylase inhibitor, NSD 1015, generally parallels the distribution of the catecholamines while indoleamine synthesis, as measured by 5-HTP accumulation, is similar to the distribution of 5-hydroxyindoleacetic acid. It may be significant that synthesis rates for the catecholamines are especially high in various areas of association cortex.