By tracing radioactively labeled proteins transported by axonal flow, projections from area 4 to the ipsi- and contralateral neostriatum and claustrum were demonstrated in 7 monkeys. A reversed topographic organization was found on both sides for the corticoneostriatal and corticoclaustral projections. The most extensive terminal field could be observed in the putamen. In contrast, very few area 4 efferents seemed to terminate in the caudate nucleus. This suggests differential functions for the two striatal components in sensorimotor mechanisms. These unexpected results give further evidence for the superior sensitivity of the autoradiographic technique, although the limitations of the new method in delineating the injection field were noted.