The concentration and rates of synthesis of norepinephrine, dopamine and serotonin were determined by spectrophotofluorometric methods in various cytoarchitectonic areas of the cerebral cortex in 54 rhesus monkeys ranging in age from 1 day to 36 months. For most regions studied, norepinephrine levels exhibit steady increases from birth through 36 months while over the same period changes in dopamine concentration are more complex and variable, particularly in the frontal lobe. Among the 3 monoamines examined, endogenous serotonin content shows the least dramatic and most rapid development, reaching adult values between 2 and 5 months of age in most cortical regions. As a consequence of these developmental shifts, the relationship of monoamine levels in various cortical areas also changes with age. At maturity, however, norepinephrine concentration exceeds that of dopamine and serotonin in the cortex of the frontal and parietal lobes whereas serotonin levels are higher than norepinephrine in the occipital cortex. Changes in rates of synthesis of the catecholamines and serotonin generally parallel developmental changes in concentrations. The greatest increments in catecholamine synthesis occur in prefrontal and posterior association cortices. Smaller but significant increases in serotonin metabolism were measured in the parietal and visual cortex between birth and 36 months while in other areas of the cortex, age-related changes in serotonin synthesis were negligible. A consistent finding at all ages is that the distribution of catecholaminergic synthesis varies inversely with that of serotonergic synthesis, indicating substantial interaction in the regulation of the two cortical systems. The present findings demonstrate that in the rhesus monkey development of monoaminergic storage capacity and synthetic processes: (1) continues over a period of months and years; (2) is generally more rapid for serotonin than for catecholamines; and (3) varies greatly in different cytoarchitectonic regions of the cerebral cortex.