Analysis of 46 cases of Wernicke's encephalopathy showed that the pathological processes in the thalamus and inferior olives were clearly different from those in the mammillary bodies and the subependymal structures along the third and fourth ventricles and the aqueduct. The latter regions showed a progressive destruction of the neuropil with severe endothelial swelling and a definite sparing of the neurones. In the thalamus and inferior olives, by contrast, the reverse picture was found with neuronal disintegration, sparing of the neuropil, and a mild endothelial swelling. The disintegrating neurones in the thalamus and olives resembled those seen in anoxic necrosis. However, careful comparisons indicated a gradual drop out of the neurones in Wernicke's encephalopathy in contrast to the sudden and simultaneous affection of the neurones in anoxia. Furthermore, the nuclear structure of the affected neurones was better and longer preserved in the Wernicke lesions. The disintegrating neurones in Wernicke's disease were removed by neuronophagia and, in severe cases, the process led to a widespread neuronal loss in the thalamus and olives. The two types of histological processes largely paralleled each other in time and severity and it is suggested that they had the same aetiology.