In the past few years the technique of mapping pathways in the central nervous system by anterograde axoplasmic transport of radioactive molecules has come into wide use and is now an important supplement to Nauta degeneration methods e,6,1°,13. Several investigatorsa,3,v,9,12 have noted radioactive substances in the postsynaptic ceils, which suggests that these cells take up labeled material released from the terminals 1,3,7. Grafstein 4 was the first to explore the possibility of tracing a pathway beyond the terminals of the initially labeled cells, by examining also the projections of the recipient postsynaptic neurons. By radiochemical measurements and by autoradiography, it was shown that after injection of [3H]proline and [aH]fucose into one eye of a mouse the contralateral striate cortex was more heavily labeled than the ipsilateral, and that the label was concentrated in layer IV 5,14. In the macaque monkey the geniculostriate pathway terminates in a very dense, highly localized manner, mainly in layer IV C. Furthermore, projections from the 2 eyes end in a characteristic alternating stripe-like pattern of oculardominance columns s. It occurred to us that if radioactive substances were transported transneuronally, injection of labeled material into one eye followed by autoradiography of the cortex might reveal the entire system of ocular-dominance columns. For the autoradiographic study of transneuronal transport in the primate visual system, 50/~1 of a saline solution containing L-[6-3H]fucose (2.5 mCi/ml, 13.4Ci/ mmole) and L-[3H]proline (7.5 mCi/ml, generally labeled, 6.8 Ci/mmole) was injected into the vitreous of the left eye ofa 3 kg normal Rhesus macaque. This injection was repeated 5 times at 12 h intervals (total dose 3.0 mCi). The animal was perfused with 10 ~ formalin 3 weeks after the initial injection. The lateral geniculate nucleus was cut into 20 #m frozen sections, and the striate cortex was embedded in paraffin and cut into 15/~m sections. The sections were coated with Ilford K5 emulsion, left in the dark for 2-4 months and developed in Dektol. Sections were counter-stained with thionin. In the lateral geniculate nucleus all layers receiving projections from the injected eye were strongly labeled (Fig. l A and B). The other layers showed grain counts higher than background, perhaps partly because of fibers of passage from the retina,