Techniques utilizing the retrograde transport of horseradish peroxidase and lesion-induced degeneration have been used to identify neurons which form the callosal projection pathway and callosal axon terminals in the primary motor cortex of the mouse. Synapses occur between callosal projection neurons and degenerating callosal afferents, demonstrating a direct synaptic link between neurons which project to homotopic regions of the contralateral cortex.