Because the axons of retinal ganglion cells are the sole channels carrying information from the eye, the organization of their central projections is important in visual processing. However, their detailed destinations and patterns of synaptic distribution at the level of single, functionally identified cells are not known. Most anatomical studies involve populations of cells or fibres and do not examine their physiological properties; physiological studies involving intracellular recording and injection of marker substances into cell bodies of single cells do not reveal distant axon terminals because the markers stain the fibres for only a few millimetres from the perikarya. To examine the central projections of retinal ganglion cells we have impaled single optic tract fibres near their sites of termination and injected them iontophoretically with the marker enzyme horseradish peroxidase (HRP). We now report that this method has revealed the thalamic and midbrain ramifications of single physiologically characterized axons. The individual optic-tract fibres branch repeatedly, sending collaterals to the superior colliculus (SC), the medial interlaminar nucleus (MIN), and to one or more laminae within the dorsal lateral geniculate nucleus (LGNd). In different nuclei the single axons form arborizations of characteristically different shapes and distribute their synaptic terminals in columns (LGNd), sheets (MIN) or widely spread patches (SC).