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Agricultural and Food Research Council, Institute of Animal Physiology and Genetics Research, Babraham, Cambridge.

1. The present results describe how a new technique of whole-tissue cytochemistry can be combined with automatic scanning of microdissected villi to measure the capacity of individual villi to hydrolyse disaccharides in different parts of the small intestine. 2. Intact villi from the mouse proximal jejunum are found to be eight times more effective than ileal villi in hydrolysing 2-naphthyl-alpha-D-glucoside, an artificial substrate for enzymes normally hydrolysing sucrose, maltose, isomaltose and trehalose in adult intestine. Homogenates of jejunal scrapings are four times more effective than ileal homogenates in hydrolysing this substrate. This discrepancy arises from relating enzyme activities to homogenate protein in cases where intestinal structure changes. 3. The eightfold difference in villus alpha-glucosidase activity is associated with a threefold difference in villus surface area. This discrepancy in turn reflects changes in the capacity of individual enterocytes to express alpha-glucosidase during migration along the crypt-villus axis. These results emphasize the futility of trying to gauge intestinal function from measurement of intestinal structure. 4. Differences between ileal and jejunal villus alpha-glucosidase activities have been further partitioned into those depending on villus structure and those depending on enterocyte development. Present results are discussed in relation to the ability of luminal nutrition to maintain a proximal-distal gradient of digestive enzyme function in the small intestine. The general applicability of this method of analysis to other studies of adaptive response is also emphasized.