Ultrastructural findings in the brains and livers of four calves affected with citrullinaemia due to a hereditary deficiency of the urea cycle enzyme arginosuccinate synthetase are described. The calves were affected in the 1st week of life with severe neurological disease characterised by depression, head pressing, stupor, convulsions and coma, accompanied by marked elevation of plasma citrulline and increasing plasma ammonia levels. Lesions included mild to moderate diffuse astroglial oedema in the cerebrocortical grey matter, and mild to severe hepatocellular hydropic change. The onset of the severe neurological signs was correlated with increasing levels of plasma ammonia, and the cerebral lesions were considered consistent with a bovine hyperammonaemic encephalopathy.