The Golgi technique has been used to examine the morphology of neurons within tectal transplants. Embryonic tectal tissue was transplanted to the midbrain of newborn rats. Four to eight months later, host animals were decapitated under anaesthesia, the unfixed brains removed and processed by Golgi-Cox impregnation. In tectal grafts, different types of neuron were recognized on the basis of the size and shape of their somata and the morphology of their dendritic trees. Neuronal types found in transplants resembled cell classes found in normal rat superior colliculus (SC). Neurons characteristic of the superficial collicular layers such as marginal, ganglion type I, stellate and horizontal cells and multipolar cells typical of the deeper collicular layers were identified in the transplants. Compared with normal cells, grafted neurons often had smaller dendritic fields and fewer dendritic spines. No laminar organization was discernable in the grafts and there was commonly no preferential orientation of perikarya or dendrites. Small cells with similar dendritic morphology were sometimes found grouped together in patches within the graft neuropil. These patches resembled cytologically and histochemically distinct areas described in previous studies and may represent areas homologous to the superficial layers of normal SC.