In hepatic encephalopathy the brain lesions are usually charaderised by polymicrocavitation, preferentially in the white matter, and the occurrence of Alzheimer type 11 cells. This paper describes an unusual manifestation of hepatic encephalopathy in two Irish wolfhound siblings in which the white matter was not involved predominantly. Both puppies had developed progressive neurological disturbances and signs of blindness. Histologically, there were widespread spongiform changes in the neuropil and fibre bundles interspersed within the grey matter, and there were some neuronal vacuoles. In both animals, the regions of the brain mainly affected were the nucleus caudatus, amygdala, cerebellar nuclei, mesencephalon, thalamus, hypothalamus and medulla oblongata. An astrogliosis characterised by Alzheimer type Il-like cells was also observed. Electron microscopy revealed a splitting of the myelin sheath. No infectious agents such as rabies virus, canine distemper virus or prion proteins were detected. The main findings in the portal regions of the liver consisted of a dilatation of the lymphatic vessels and increased numbers of small arteries, indicating that a portosystemic shunt was the probable cause of the spongiform brain lesions.