The concentrations and contents of immunoreactive substance P (SP), neurokinin A (NKA), vasoactive intestinal polypeptide (VIP) and gastrin releasing peptide (GRP) were measured in acid-ethanol extracts of intestine (duodenum-jejunum-ileum) and pancreas of C57BL/KsJ diabetes-obese (db/db) mice, Aston obese-hyperglycaemic (ob/ob) mice, and their respective lean controls. The intestinal concentration of GRP and pancreatic concentrations of VIP and GRP were 36-57% lower in lean Aston mice than lean C57BL/KsJ mice, indicating the influence of genetic background in control mice. Intestinal concentrations of SP and NKA were reduced by 19-33% in the db/db and ob/ob mutants compared with their lean controls, but the intestinal contents of these peptides were normal or greater than normal due to intestinal hypertrophy of the mutant mice. The intestinal VIP concentration was not altered, but the content was increased by 87% and 25% respectively in db/db and ob/ob mice, whereas the intestinal GRP concentration was reduced by 51% in ob/ob mice. Pancreatic concentrations and contents of NKA, VIP and GRP were similar in lean and db/db C57BL/KsJ mice. However, pancreatic concentrations and contents of VIP and GRP were reduced by 51-55% in ob/ob mice compared with their lean controls. The sensitivity of the present assay did not permit accurate determination of the low pancreatic concentrations of SP. The results suggest that the spontaneous ob/ob and db/db syndromes of obesity and diabetes in mice are associated with reduced intestinal concentrations of SP and NKA. The ob/ob mouse also exhibited reductions of intestinal GRP and pancreatic GRP and VIP concentrations. These changes in regulatory peptides may relate to abnormalities of intestinal and possibly pancreatic function in obese and diabetic mutant mice.