The distribution of axonal spheroids was examined in the central nervous system of gracile axonal dystrophy (GAD) mutant mice. Only few spheroids are observed in the gracile nucleus of the medulla in normal mice throughout the period examined, while they are first noted in GAD mice as early as 40 days after birth. The incidence of spheroids shifts from the gracile nucleus to the gracile fasciculus of the spinal cord with the progress of disease, suggesting that the degenerating axonal terminals of the dorsal ganglion cells back from the distal presynaptic parts in the gracile nucleus, along the tract of the gracile fasciculus, toward the cell bodies in the dorsal root ganglion. This phenomenon indicates that the distribution of spheroids is age dependent and reflects a dying-back process in degenerating axons. In addition to the gracile nucleus and the gracile fasciculus, which is one of the main ascending tracts of primary sensory neurons, it was noted that the other primary sensory neurons joined with some of the second-order neurons at the dorsal horn and neurons at all levels of the dorsal nucleus (Clarke's column) are also severely affected in this mutant. The incidence of the dystrophic axons are further extended to the spinocerebellar tract and to particular parts of the white matter of the cerebellum, such as the inferior cerebellar peduncle and the lobules of I-III and VIII in the vermis. These results indicate that this mutant mouse is a potential animal model for human degenerative disease of the nervous system, such as neuroaxonal dystrophy and the spinocerebellar ataxia.