The objective of this study was to identify the components involved in a local synaptic circuit in the mouse cerebral cortex. The local axon collaterals of corticothalamic (CT) projection cells in the posteromedial barrel subfield area of primary somatosensory cortex were labeled by the retrograde transport of horseradish peroxidase injected into the ipsilateral thalamus. Thalamocortical (TC) axon terminals in the same region of cortex were labeled by lesion induced degeneration. CT axon terminals synapsed preferentially with dendritic shafts, whereas TC axon terminals synapsed mainly with dendritic spines. Some dendrites received both CT and TC synapses. Dendrites were interpreted to belong to nonspiny multipolar cells. These results indicate that a reciprocal synaptic relationship exists in the cortex between nonspiny multipolar cells and CT projection cells. Both CT projection cells and nonspiny multipolar neurons have been shown previously to receive TC synapses (White and Hersh: J. Neurocytol. 11:137-157, '82; White, Benshalom, and Hersch: J. Comp. Neurol. 229:311-320, '84). These findings imply that a triadic relationship involving afferent input and populations of CT projection and intrinsic neurons is a basic feature of the synaptic organization of the cerebral cortex.