The forebrain circuits involved in coordinating the bilateral movements of the whiskers have been elucidated by a series of neuronal tracing studies aimed at characterizing the interhemispheric connections of the primary motor (MI) cortex. In some experiments, different anterograde tracers were placed in the MI whisker region of each hemisphere. In other experiments, an anterograde tracer injection in one MI whisker region was paired with a retrograde tracer injection in the opposite hemisphere. In the last study, a retrograde tracer was placed in one of the forebrain targets of MI, thereby revealing the bilateral distribution of MI neurons that project to the injected target. The relative strengths of the interhemispheric connections of MI cortex were quantified by a variety of methods. In addition to measuring the density and spatial extent of terminal and somal labeling, we analyzed the amount of tracer overlap produced when different tracers are injected in the MI regions of both hemispheres.