In a light and electron microscopic study of the substantia nigra of the rat, the distribution and morphology of nigrotectal neurons and the pattern of termination of striatonigral fibres have been examined following the placement of horseradish peroxidase injections in the superior colliculus and kainic acid lesions in the dorsal striatum. In confirmation of previous findings, nigrotectal neurons which had been identified by the retrograde transport of horseradish peroxidase from the superior colliculus had mainly medium sized somata, varied from fusiform to stellate in shape and were found in mainly ventral regions of the rostral two-thirds of the substantia nigra pars reticulata. On electron microscopic examination, single and multiple (from two to six) degenerating striatonigral boutons were found in synaptic contact with the soma, proximal mainstem dendrites and small dendrites (but mainly on small dendrites) of labelled nigrotectal and unlabelled nigral neurons in the ventral region of the pars reticulata. In addition, a small number of degenerating striatonigral boutons formed axoaxonic synapses with degenerating or normal boutons which were presynaptic to nigral dendrites. Almost all of the identified striatonigral synapses were of the symmetrical type, although a few degenerating boutons established asymmetrical synaptic contacts on unlabelled dendrites. These findings provide evidence of a monosynaptic input from the dorsal striatum to nigrotectal projection neurons in the substantia nigra and thus demonstrate the existence of a bineuronal pathway from the striatum through the substantia nigra to the superior colliculus. The possible significance of the pattern of termination of striatonigral fibres in the substantia nigra is discussed with reference to the known dendritic arborization of nigral neurons.