1. Immunohistochemical methods were used to identify dopaminergic, noradrenergic, cholinergic and serotonergic axons in cynomolgus monkey prefrontal cortex. 2. The densities of dopaminergic and noradrenergic systems varied in a similar fashion across prefrontal cortical regions, whereas the densities of cholinergic and serotonergic axons were regionally homogeneous. 3. In contrast, the laminar distributions of labeled axons were very different among these four systems. 4. The different patterns of organization of axons arising from these four afferent systems indicate that they may differ substantially in their regulation of prefrontal cortical function.