1. The functional properties of the ileal mucosa following jejunal resection or interposition in the jejunum were studied with the help of membrane vesicles. 2. Despite the development of functional and morphological features in the mucosa which are more generally associated with the jejunum than with the ileum, examination of brush-border vesicles derived from ileal and jejunal enterocytes from resected or transposed intestines reveals that the functional characteristics of apical membranes isolated from the different regions of the intestine are maintained after these surgical manoeuvres. Vesicles from control ileum develop an 'overshoot' uptake of glucose that is 3-4 times smaller than that of jejunal vesicles, a difference that is still observed in membranes prepared from mucosa of ileal loops following proximal resection or interposition in the jejunum.