In the clinical setting, resection of the ileumresults in an inferior functional outcome compared tojejunal resection. This may be related to a greateradaptive capacity of the ileum, intrinsic structural and functional differences, or regionaldifferences in motor and hormonal function. Our aim wasto evaluate the relative contributions of these factorsto functional outcome after resection of the proximal or distal intestine. Twenty-four dogs underwenteither intestinal transection or 50% resection of theproximal or distal intestine. Studies (nutritionalstatus, absorption, adaptation, motility, peptide levels) were performed every four weeks untilthe animals were killed at 12 weeks. Caloric intake wassimilar in all four groups. Weight loss was greater andmore sustained after distal resection (DR). Serum cholesterol levels decreasedsignificantly only in the DR group. While stool weightand moisture were similar, the DR animals hadpersistent, significant steatorrhea. Intraluminalanaerobic bacteria and SCFA concentrations were significantlygreater in the ileum but were not influenced byresection. Intestinal remnant length increased to agreater extent after proximal resection (PR), butcircumference increased to a similar extent after bothresections. Villus height and crypt depth increasedsignificantly only after PR. MMC frequency was similarin all four groups. In the DR animals 26% of migrating motor complexes (MMCs) originated within theremnant. The jejunal remnant of these animals had adominance of cluster activity similar to the intactdistal ileum. Following PR, the postprandial motilin response was decreased. After DR, there weretransient increases in neurotensin and PYY. Of thevarious factors evaluated, mucosal adaptation and theintestinal motor response appear most likely to explain the inferior nutritional and absorptive outcomeassociated with resection of the distal smallintestine.