The corticocortical afferents innervating the prefrontal cortex in the monkey were studied by means of the retrograde axonal transport of horseradish peroxidase. After injection of small amounts (0.3-0.5 microliter) of this enzyme into various parts of the prefrontal cortex, many labeled neurons (mostly pyramids of 15-25 microns in diameter) were found in various cortical regions of the ipsilateral hemisphere. A small part of the prefrontal cortex received fibers from other parts of the same cortex. For example, area 8 receives many fibers from both the rostral part of area 9 and a small area adjacent to the inferior branch of the arcuate sulcus. On the other hand, area 9 in the inferior prefrontal convexity receives fibers from localized parts of areas 8 and 9 in the dorsolateral convexity as well as from area 6. It is also apparent that association connections from the dorsolateral to the inferior convexity are stronger than those going in the opposite direction. The prefrontal afferents from other cortical regions include many fibers originating from the posterior association cortex as well as some fibers arising in the cingulate and orbital gyri. The prefrontal cortex does not receive direct corticocortical fibers from the motor and "primary" sensory cortices. There is a topographic pattern in the prefrontal projections from the cortical walls (STs area) surrounding the superior temporal sulcus. Thus, the caudal half of the STs area projects to area 8 and a small adjacent part of area 9. The dorsal wall of the rostral half of the STs area projects to areas 9-12, the fundus to the inferior convexity, and the ventral wall only to the caudal part of the convexity. Projections from the circumjacent association cortex of the STs area to the prefrontal cortex as well as to the STs area are likewise found to be topographically organized. This suggests that certain parts of the posterior association cortex projecting to particular areas of the prefrontal cortex, also send fibers to those parts of the STs area which project to the same prefrontal areas.