We have investigated the possible benefits of elemental diets, especially a diet supplemented with L-glutamine, on maintenance of intestinal absorptive function in rat small intestine damaged by 5-fluorouracil. Although a standard rat diet sustained better body growth in control rats, each of the elemental diets and the diet containing intact casein in place of hydrolyzed casein was beneficial in promoting less body weight loss during the 3 d after 5-fluorouracil injection. The same significant benefit was seen in absorptive activity measured in small intestine in vitro 3 d after the cytotoxic injury. Glutamine supplementation, however, did not confer any significant advantages, although it did cause significant elevation of muscle glutamine pools. This elevation was substantially less than the corresponding increase in muscle glycine content after dietary supplementation with glycine.