[en] The insulin-degrading activity and insulin content of aqueous extracts of rat fecal material isolated from the proximal colon were measured by radioimmunoassay in the presence and absence of various inhibitors of proteolysis. In the absence of protease inhibitors, up to 40% of the [¹²⁵I]-insulin present in radioimmunoassay tubes was degraded to a form that was not precipitated by trichloroacetic acid. In the presence of bacitracin, EDTA or a mixture of protease inhibitors, insulin degradation was diminished to < 10%. The concentration of insulin in aqueous extracts of rat proximal colon fecal material was found to be small but significant, ranging from 10-120 μU/g wet weight when measured in the presence of protease inhibitors. The efficacy of various protease inhibitors differed when added to the aqueous fecal extract than when added to rat serum suggesting that a distinct insulinase activity was present in rat feces. Degradation of exogenously added [¹²⁵I]-insulin by the aqueous fecal extract was time- and temperature-dependent. Fecal samples obtained from streptozotocin-diabetic rats contained reduced amounts of insulin, suggesting that the fecal insulin was produced partially, but not exclusively, in the pancreas. These results indicate that fecal material in the rat proximal colon contains small amounts of insulin, as well as a rich supply of insulinase activity, and that insulin receptors located on the mucosal surface of proximal colon epithelial cells may be exposed to insulin in vivo