We have studied the intrinsic organization of macaque striate cortex by tracing the pattern of horseradish peroxidase (HFiP)-labeled axons and cell bodies produced by microinjections of HRP into single cortical laminae. Both anterograde and retrograde transport results were used to examine: (1) the pattern of projections from lamina 4C to the superficial layers; (2) the projection from lamina 4C to deeper cortical layers; and (3) the projections to lamina 4C from other cortical laminae.Laminae 4C0 and 4Cfi differ in their pattern of projections to the superficial layers of striate cortex. Axons from neurons in lamina 4C/3 ascend through lamina 48 without giving off collaterals and terminate in lamina 4A and in the base of lamina 3. By contrast, axons from neurons in lamina 4Ccu terminate in lamina 48 and less densely in the 4A/3B region.The projection from lamina 4Cp to lamina 4A is particularly dense and is distributed in a patchy fashion immediately above each injection site. The projection from lamina 4Cp to lamina 38 appears less dense and more widespread; we estimate that individual 4Cj3 axons may spread laterally for more than 400 pm. Furthermore, the pattern of HRP-labeled cell bodies in lamina 4Cj3 following injections into laminae 4A and 3B provides evidence for a subdivision within SC& These injections always produce a large number of labeled neurons in the upper part of lamina 4C& whereas the lower portion contains few labeled neurons that are located immediately under the center of the injection site.Both lamina 4& and lamina 4Cp also contribute less dense projections to the deeper layers of cortex. Lamina 4Cj3 projects mainly to lamina 6, whereas lamina 4Ca contributes axon terminals to both lamina 5A and lamina 6.Neurons in lamina 6 provide the bulk of the intracortical projections to lamina 4C. These results confirm or refine many conclusions about intrinsic connections of striate cortex drawn from Golgi material and suggest new patterns of connections not suspected from previous work.The key to understanding cortical function lies in establishing the way in which inputs from the thalamus and other brainstem regions are transformed into the response properties of output neurons. Although we are still far from understanding the nature of this complex process, it is clear that the achievement of this objective will require a detailed knowledge of intracortical circuitry and the relation of this circuitry to afferent and efferent cell groups.The macaque striate cortex provides an excellent system for the study of intracortical circuitry because of the remarkable degree of lamlnar segregation of different afferent and efferent pathways. For example, inputs from the magnocellular and parvocellular laminae of the lateral geniculate nucleus (LGN) terminate in separate and largely nonoverlaping tiers within lamina 4C with magnocellular afferents occupying a more superficial position (lamina 4Ca) than parvocellular ones (lamina 4Cp) (Hubel and Wiesel, 1972; Hendrickson et al., 1978). Likewise, the...