The study of the temporospatial pattern of lower motor neuronal generation in the wr/wr mouse demonstrated that the pathological changes started in the neuronal perikaryon in the spinal cord. Thus the wr/wr mouse suffers from a primary motor neuronopathy. Axonal pathology predominated at the level of the ventral root and suggested a focal proximal axonopathy. Microtubular segregation, neurofilamentous accumulation in the axon, and axonal or adaxonal vacuoles appeared to be closely related to subsequent axonal degeneration. Large myelinated fibres in the cervical ventral roots progressively degenerated and diminished in number with age. On the other hand, significant axonal regeneration coincided, with the axonal degeneration in the cervical ventral roots. A dying-back type of degeneration and axonal atrophy were not present. This study demonstrated that a primary disease of the perikaryon may express itself as a focal proximal axonopathy characterized by axonal degeneration and attempted axonal regeneration.