Rats maintained for a period of 16 weeks on a supplemented solid semisynthetic diet and ethanol corresponding to 35% of the total caloric intake developed ultrastructural changes in the small intestine as compared to pair-fed controls. The enterocytes from jejunal and ileal villi of the alcoholic animals exhibited conspicuous alterations of mitochondria and smooth and rough endoplasmic reticulum. The mitochondrial changes were characterized by enlargement, swelling, decreased matrical density, ruptured cristae, and occasional myelin figures. The smooth endoplasmic reticulum was dilated and apparently proliferated in a few epithelial cells. The rough endoplasmic reticulum appeared scarce with fewer ribosomes attached to the cisternae. These findings are postulated to provide further evidence that the chronic consumption of ethanol exerts a direct causative role in the pathogenesis of small intestinal damage in rats. Furthermore it can be especulated that these changes could be responsible, at least in part, for the functional abnormalities commonly found in chronic alcoholism.