Cortico-cortical interconnections within the visual cortex of squirrel monkeys (Saimiri sciureus) were studied by means of single or multiple injections of [3H]leucine or combined [3H]leucine and horseradish peroxidase into the dorsolateral striate and prestriate cortices. Injections of [3H]leucine were also made into the dorsolateral striate cortex of a Macaca speciosa for comparison. The results indicated that in Saimiri the dorsolateral prestriate cortex is precisely and reciprocally connected with the striate and peristriate cortex, and with the posterior bank of the superior temporal sulcus, including the middle temporal area (MT). The dorsolateral striate area is precisely and reciprocally connected with the prestriate and MT area, but does not project to the perstriate cortex as in the case of the macaque. At each target site, the radioactively labelled terminal fields are distributed in a distinctly columnar fashion, with labelled columns (120-480 microm in diameter, commonly 240 microm) interdigitating with sparsely labelled or unlabelled columns of lesser widths. The HRP-labelled neurons invariably exist within the radioactively labelled columns. For all of the cortical areas examined except area 17, lamina IV is the principal 'receptive' layer for associational fibers, while layers III and II, as well as V and VI within the same column, receive progressively less input. As for area 17, associational fibers from the prestriate cortex terminate mainly within lamina I, with laminae II, III, V and VI receiving decreasing amounts of input. The present results indicate that the major visual cortical areas are interconnected in precise, topographical and reciprocal fashion. The columnar arrangement is found to be the basis of both extra- and intercortical connections.