The development of synaptic interconnections between co-cultured explants of central and peripheral nervous tissue from chick embryos has been investigated by light and electron microscopy. Two sets of co-cultured explants were used: (a) dorsal root ganglion (DRG) and spinal cord and (b) retina and tectum. Both sets of co-cultured explants became linked by bundles of fibres but the most consistent results were obtained with the DRG-spinal cord explants. Thus axons from the DRG extended large distances across the culture substrate to reach and enter mainly the dorsal horn region of the spinal cord explants. In contrast retina-tectum links were less frequently established and were less extensive, possibly because there are fewer cells in retinal explants capable of establishing contacts in tectal explants than there are cells in DRG explants capable of establishing contacts in the spinal cord. In order to distinguish between synapses involving only neuronal elements within an explant and those involving ingrowing fibres, fibre bundles linking adjacent explants were transected and the preparations fixed two to six hours later. Electron microscope study of such cultures revealed degenerating neurites and terminals in the spinal cord explants receiving DRG fibres but none in the corresponding DRG explants. Retinal explants contain numerous synapses of many types but degenerating terminals could not be found within the retinal explants after nerve fibre transections. Degenerating neurites and terminals were found within tectal explants but they were fewer and more difficult to locate than those found within spinal cord explants. The reasons for such differences are discussed.