We investigated the effect of chronic mechanical compression of the cervical spinal cord on the number of spinal accessory motoneurons in 25 tiptoe-walking Yoshimura mice. The animals had calcified deposits in the atlantoaxial membrane at the C1-C2 vertebral level, compressing the spinal cord posterolaterally. Motoneurons of the spinal accessory nerve between C1 and C5 segments were labelled using wheat germ agglutinin-horseradish peroxidase (WGA-HRP) injected into the sternocleidomastoid muscles. The counted cells were processed into a three-dimensional computer display to analyse the cytoarchitectonic changes caused by external cord compression. The number of WGA-HRP-labelled spinal accessory motoneurons was significantly reduced on the affected side. The number of motoneurons in compromised C2 and C3 cord segments correlated linearly with the extent of mechanical compression, but no such relationship was present on the contralateral side. There was an increase in the number of WGA-HRP-labelled spinal accessory motoneurons in the medial cell pools of the anterior grey horn at a level most rostral to the compression, and in the ventrolateral cell pools at levels immediately rostral to the compression. Our findings suggest that the spinal accessory motoneurons translocate rostral to the area of external compression in order to avoid mechanical injury.