In order to determine an advantageous site for intestinal insulin absorption, the hypoglycemic effects of insulin after administration to the duodenum, the jejunum, the ileum and the colon were investigated using an in situ loop method. Insulin solution was administered to the various loops of fasted rats with or without aprotinin (AP) as a protease inhibitor, or absorption enhancers such as sodium caprate, Na2EDTA or sodium glycocholate. An obvious hypoglycemic effect of insulin alone was seen only in the ileum loop washed with phosphate buffered saline. When coadministered with AP, the most remarkably amplified effect was again observed in the ileum. In the ileum, the area under the serum insulin levels vs. time curve from 0 to 4 h was linearly related to the logarithm of the AP dose. Both sodium caprate and Na2EDTA significantly promoted the hypoglycemic effect of insulin at all sites, and their intensity increased towards the distal regions of the intestine. On the other hand, sodium glycocholate improved only colonic insulin efficacy. These results suggest that the ileum seems to be the most useful region in the small intestine for insulin absorption; however, insulin must be protected from proteolysis to enhance its absorption. In addition, the insulin efficacy could be increased by absorption promoters more effectively in the colon than in the small intestine.