In an attempt to obtain information about changes of neurofilaments in motor neurons after axotomy, we immuno-histochemically investigated the accumulated neurofilaments in the dorsal motor nucleus of the vagal nerve, which shows nerve cell loss and degenerative changes after axotomy, and in the hypoglossal nucleus, which shows regenerative changes. Affected neurons in the hypoglossal nucleus showed intensified immunoreactivities for neurofilament antibodies phosphorylated at the carboxy-terminal, and these reactions disappeared with trypsin treatment. Accumulated neurofilaments in the neuronal perikarya in the dorsal motor nucleus of the vagal nerve and axons in brain stem also showed intensified immunoreactivities for the same antibodies, and these reactions remained positive after trypsin treatment. Anti-ubiquitin antibody preferentially stained accumulated neurofilaments in the affected vagal neurons, while no reaction was found in the affected hypoglossal neurons. Phosphorylated neurofilaments in hypoglossal neurons are vulnerable to trypsin treatment probably because of the blocking of polymerization or the disassembly of neurofilaments due to amino-terminal phosphorylation. In vagal neurons, the deteriorated amino-terminal phosphorylation or hyperphosphorylation at the carboxy-terminal seems to cause the cross-linkage and polymerization of neurofilaments, and densely packed polymerized neurofilaments probably fail in axonal transport resulting in nerve cell degeneration and death in the dorsal motor nucleus.