1. Water absorption and cytoplasmic dipeptidase activities were determined in isolated rat small intestine after animals had fed on various ‘elemental’ diets or a standard rat diet and after a single injection of 5-fluorouracil.
2. Vivonex diets were associated with a decrease in dry weight of the small intestine and an increase in glucose and water absorption in vitro.
3. The nature of the dietary nitrogen profoundly affects the activities of intestinal peptidases.
4. Water absorption was severely depressed 3 days after 5-fluorouracil administration: the extent of depression was independent of the diet. Thus, any ameliorative effects of ‘elemental’ diets on 5-fluorouracil toxicity are unlikely to be mediated by direct protection of intestinal absorption.