One theory of compensatory hypertrophy of the gut suggests that ileal growth after jejunectomy results from exposure of the ileum to villus-enlarging factors emanating from the papilla of Vater. This hypothesis was tested by examination of the ileum after bypass of pancreatobiliary secretions to the colon. Hypertrophy of the ileum occurred after esophagoileostomy and gastroduodenojejunal exclusion, suggesting that adaptive growth of the gut depends on additional factors. There is evidence to support several theories of compensatory hypertrophy. A hypothesis seeking to reconcile the conflicting notions of intraluminal and hormonal stimuli is suggested.