Although cells in layer 4 of cat striate cortex represent the first stage in the cortical processing of visual information, they have considerably more complicated receptive field properties than the afferents to the layer from the lateral geniculate nucleus. In considering how these properties are generated, we have focused on the intrinsic cortical circuitry, and particularly on the projection to layer 4 from layer 6. Layer 6 pyramidal cells were injected with horseradish peroxidase and examined at the light and electron microscopic level. The labeled axon terminals were found to form asymmetric synapses and to show a strong preference for contacting dendritic shafts. Serial reconstruction of dendrites postsynaptic to labeled layer 6 cell axon terminals showed that a large proportion of the postsynaptic dendrites belonged to smooth and sparsely spiny stellate cells, suggesting a selective innervation of these cell types. In contrast, the geniculate projection to layer 4 made synapses primarily with dendritic spines and, as a result, the large majority of terminals ended on spiny cells. Since smooth and sparsely spiny stellate cells are thought to mediate inhibition within the cortex, we suggest that one effect of the layer 6 to layer 4 projection could be to contribute to inhibitory features of the receptive fields of layer 4 cells.