Luminal nutrients exert a powerful trophic effect on small bowel mucosa. Recent evidence suggests that a circulating factor, possibly enteroglucagon, is also growth-promoting. In order to study the isolated effect of nonluminal influences on bowel mucosa, Thiry-Vella fistulae (TVF) were constructed in rats. Circulating enteric hormone concentrations were manipulated by resecting different lengths of remaining gut. Thirty-two male Wistar rats had either 25%, 50%, 75%, or 90% proximal small bowel resection. In each animal the first 25% of resected bowel was exteriorized as a Thiry-Vella fistula. Seven control rats underwent jejunal transection. Twelve days postoperatively the fasted animals were killed, and circulating and tissue concentrations of enteroglucagon and CCK were estimated by radioimmunoassay. Crypt-cell production rate was used as an index of cellular proliferation in the Thiry-Vella fistulae. Proximal small bowel defunctioned in the Thirty-Vella fistulae had a significantly lower crypt-cell production rate and enteroglucagon and CCK content than the equivalent segment in transected rats. Further small bowel resection produced a subsequent increase in circulating enteroglucagon and CCK concentrations, an increase in the Thiry-Vella fistula content of these hormones, and a doubling of the crypt-cell production rate in the Thiry-Vella fistulae. These results show that circulating enteroglucagon and CCK concentrations match closely with enterocyte production even when luminal influences are excluded. It is suggested that circulating factors may play a major role in postresectional ileal hyperplasia. This hyperplasia apparently affects endocrine cells as well as enterocytes.