Dendro-dendritic synapses were studied in serial sections of rat substantia nigra in subjects pretreated with intraventricularly administered 5-hydroxydopamine, a monoaminergic synaptic label. These synapses had symmetrical membrane thickenings and small clusters of heavily labeled pleomorphic vesicles in the presynaptic dendrites, which were assumed to extend from the dopaminergic cells of pars compacta. Dendro-dendritic synapses were seen in the pars compacta of the substantia nigra, but were not found in blocks trimmed to include only the nondopaminergic pars reticulata. The presynaptic dendrites did not receive other synapses near the sites of dendro-dendritic synapses. These dendrites were frequently apposed to other dendrites for long distances with no intervening glial processes. In many cases, several adjacent dendro-dendritic contacts were made by a single presynaptic dendrite onto several different postsynaptic dendrites. Presynaptic dendrites did not participate in reciprocal synapses, serial synapses, or dendro-axonic synapses. Presynaptic and postsynaptic dendrites engaged in dendro-dendritic synaptic contact were of similar appearance and both had cross-sectional diameters of 0.23-1.9 microns. In several cases, label could also be detected in the postsynaptic dendrite in cisternae of smooth endoplasmic reticulum, providing evidence for dendro-dendritic synapses between dopaminergic neurons. Release of dopamine from dopaminergic dendrites and their role in the control of neuronal activity in substantial nigra are discussed.