Rats were maintained 16 weeks on a well-balanced semisynthetic solid diet supplemented with ethanol which comprised 35% of total calories. A control group was pair-fed the same basic diet with sucrose replacing ethanol isocalorically. Striking changes in mucosal morphology and mitotic index were observed in the jejunum and ileum of ethanol-fed rats in comparison to pair-fed controls. Furthermore, it is significant that these changes were more pronounced in the ileum than in the jejunum. Since ethanol is almost completely absorbed in the stomach and upper intestine, under the conditions of this study, we propose that, apart from a possible topical toxic effect of ethanol, there appear to be other separate possible causes of the extensive small intestinal changes found in ethanol-fed rats. The first is that the changes are due to injurious effects of blood-borne ethanol; secondly, the changes could be a functional adaptation due to altered luminal nutrition as a consequence of the introduction of ethanol in the diet.