Carbocyanine fluorescent dye, DiI, is an excellent anterograde/retrograde neural tracer, but its efficacy for the anterograde labeling of neural circuits in the adult brain tends to decrease with ages. The present study shows that an injection of DiI into the motor cortex of the young adult jimpy mutant mice (Plp1(jp)/+) resulted in successful anterograde labeling of corticospinal tract fibers. Furthermore, an injection of Fast Blue into the lumbar spinal cord of the mutant mice resulted in retrograde labeling of layer 5 corticospinal tract neurons within the motor cortex. Since no abnormality except for myelin deficiency is known in the long descending and ascending tracts of jimpy mutant mouse, this mutant is suitable for neural tracing studies of long axonal trajectories with the use of carbocyanine dye, DiI, although these males die between 20 and 40 days of age.