Thirty Brown Swiss cattle, 7 to 30 months old, clinically affected with the weaver syndrome and five clinically normal Brown Swiss were studied. Ultrastructural examination mainly of the thoracic spinal cord revealed axonal changes accompanied by degeneration of myelin sheaths that were fragmented and lamellae vesiculated. The degeneration was accompanied by moderate gliosis and phagocytosis of degenerated myelin. There were no inflammatory changes. The ultrastructural changes may represent a disturbance of axoplasmic transport and subsequent axonal degeneration.