The objective of this study was to examine synaptic relationships among 3 neuronal elements in the cerebral cortex: thalamocortical afferents (TC), corticothalamic projection cells (CT), and GABAergic neurons. TC axon terminals in the barrel cortex of the mouse were labeled by lesion induced degeneration; local axon collaterals belonging to CT cells were labeled by the retrograde transport of horseradish peroxidase; and GABAergic neurons were identified using immunocytochemistry. CT and GABAergic neurons form synapses with each other and both receive synapses from TC afferents. These findings indicate the existence in the cerebral cortex of a triadic circuit involving afferent input both to projection and to local inhibitory neurons, and reciprocal synaptic interactions among these neuronal populations.