Gastric inhibitory polypeptide (GIP) is produced within endocrine cells of the small intestine and released into the circulation upon nutrient ingestion. This study has quantified the levels of this insulinotropic peptide in the intestines of lean and diabetic obese ob/ob mice and estimated the proportion that is glycated. The total intestinal GIP concentration and content of the diabetic mice were significantly greater (p < 0.01) than that of control animals. Affinity chromatographic separation and side-viewing GIP radioimmunoassay demonstrated that approx 20% of the GIP extracted from intestines of ob/ob mice was present in glycated form. Less than 2% of intestinal GIP was glycated in lean mice. In conclusion substantial quantities of glycated GIP exist within the intestines of diabetic ob/ob mice, suggesting that this may be a contributing factor to the physiological disarray of this syndrome.