1. A brief review of the literature concerning the central nervous system involvement in morphinism is presented. Although there are scattered clinical and pathological reports which indicate that acute and chronic residuals may result, no consistent picture of the neurological involvement can be formulated.2. Histopathological studies are reported in three cases of morphinism in which the illness varied in duration from the very acute to the chronic stage.3. Definite histological alterations were present within the brains of all cases and consisted of acute or chronic neuronal alteration and destruction and irregular penvascular demyelinization. The damage was similar in all cases but was more apparent in the chronic ones. The most severe nerve cell changes were observed in the medulla.4. There is good experimental evidence to suggest that morphine at certain doses may be a cortical stimulant and is probably contraindicated in toxic conditions resulting in convulsive phenomena.