To determine whether intraluminal administration of epidermal growth factor (EGF) has a trophic effect on small bowel mucosa, catheters were surgically placed in the ileum of adult rats and infused with EGF. Comparing animals receiving EGF (5 micrograms/48 h) with controls, in the ileum mean mucosal ornithine decarboxylase specific activity increased by greater than 200% (p less than 0.001), mean deoxyribonucleic acid specific activity and crypt labeling index increased by greater than or equal to 100% (p less than 0.001), and mean deoxyribonucleic acid content of the mucosa increased by 25% (p less than 0.05). During these studies, the jejunum was not exposed to ileal infusate, as shown with the use of a phenol red marker. Nevertheless, all measurements except deoxyribonucleic acid content increased in the jejunum as well, although to a lesser extent. A greater rise in mucosal ornithine decarboxylase and deoxyribonucleic acid specific activity could be demonstrated in the jejunum when EGF was infused directly into this segment. Mucosal ornithine decarboxylase activity was found to be dose-dependent and to increase in the ileum only after a latent period of 12-24 h. We conclude that intraluminal administration of EGF stimulates a mucosal proliferative response in the small intestine. Intraluminal EGF appears likely to be one of a number of endogenous trophic factors in the small bowel.