The distribution of tyrosine hydroxylase (TH)-immunoreactive fibers was characterized immunohistochemically in the prefrontal cortical regions of both Old World cynomolgus monkeys (Macaca fascicularis) and New World squirrel monkeys (Saimiri sciureus). In both species, differences in the density and/or laminar distribution of TH-labeled fibers were detected both across and within almost every prefrontal cytoarchitectonic region. In cynomolgus monkeys, areas 9 and 24 had the greatest density of TH-labeled fibers, areas 11, 12, 13 and 25 were of intermediate density, and areas 10 and 46 had the lowest density of immunoreactive fibers. Differences in fiber density within many of these regions were also consistently observed. On a laminar basis, the distribution of labeled fibers in a given area of cynomolgus prefrontal cortex was systematically related to the overall fiber density of that area. For example, in the lightly innervated fundus of the principal sulcus (area 46), labeled fibers were primarily present in layer I and layers V-VI, whereas in area 9, the most densely innervated region, TH-labeled fibers were present in all cortical layers. Similar regional differences in the density and laminar distribution of TH-immunoreactive fibers were also present in squirrel monkey prefrontal cortex. In previous studies, we have analyzed the regional and laminar distributions of fibers immunoreactive for TH and dopamine-beta-hydroxylase (DBH), a specific marker for noradrenergic cortical fibers, in multiple areas of cortex from both normal and locus ceruleus-lesioned animals. These comparisons, which have been confirmed in the present report, indicate that anti-TH and anti-DBH label distinct populations of axons in monkey neocortex, which presumably are dopaminergic and noradrenergic, respectively. Thus, the distribution of TH immunoreactivity described in the present report suggests that dopaminergic fibers are distributed in a very heterogeneous fashion in monkey prefrontal cortex. The distinctive innervation patterns exhibited by these fibers reveal the regions and layers that may be the principle sites of action of dopamine in exerting its effects on prefrontal cortical function.