Non-pyramidal cells with transcallosal projections were identified in the area 17/18 border region of the cat by retrograde transport of horseradish peroxidase injected into border region of the opposite hemisphere. From several hundred neurons filled with a Golgi-like diaminobenzidine (DAB) reaction product, seven cells were identified by their radially oriented smooth dendrites as possible non-pyramidal cells. Following thin-sectioning and examination with the electron microscope, four of the neurons proved to be layer IV spiny stellate cells with incompletely filled dendritic spines, and two proved to be layer III pyramidal cells with an incompletely labelled apical dendrite and dendritic spines. The remaining neuron was a non-pyramidal cell whose essentially smooth dendrites were covered with synapses, and whose cell body formed both symmetric and asymmetric synapses with presynaptic terminals. To better assess how many non-pyramidal cells might be labelled, thin sections of the area 17/18 border were surveyed using material processed with tetramethylbenzidine (TMB), and another five labelled non-pyramidal cells with transcallosal projections were identified by the needle-like crystals of TMB reaction product they contained. During the study it became evident that both the DAB and TMB reaction products in the lightly labelled neurons tended to be associated with granules that are 0.5 microns or larger in diameter and that had the characteristics of lysosomes. These granules are also visible in the light microscope as dark puncta. The numbers of puncta in profiles of pyramidal and of non-pyramidal cells in layers II/III and IVa of the area 17/18 border region and in the control acallosal region of area 17 were counted and compared. These comparisons revealed that labelled transcallosally projecting non-pyramidal cells may constitute 10-32% of the non-pyramidal cell population at the area 17/18 border region. Similar values were also obtained for pyramidal cells in this region. Consequently, it is concluded that significant numbers of non-pyramidal cells have axons that project through the corpus callosum to the contralateral hemisphere.