The incretins are a class of hormones released from the small bowel that act on the endocrine pancreas to potentiate insulin secretion in a glucose-dependent manner. Due to the requirement for an elevated glucose concentration for activity, the incretins, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1, have potential in the treatment of non-insulin-dependent diabetes mellitus. A series of synthetic peptide GIP fragments was generated for the purpose of elucidating the bioactive domain of the molecule. Peptides were screened for stimulation of cyclic AMP (cAMP) accumulation in Chinese hamster ovary cells transfected with the rat islet GIP receptor. Of the GIP fragments tested, GIP(1-14) and GIP(19-30) demonstrated the greatest cAMP-stimulating ability over the range of concentrations tested (up to 20 microM). In contrast, GIP fragments corresponding to amino acids 15-42, 15-30, 16-30 and 17-30 all demonstrated weak antagonism of GIP(1-42) activity. Competitive-binding displacement studies indicated that these peptides were low-affinity ligands for the GIP receptor. To examine biological activity in vivo, a bioassay was developed in the anesthetized rat. Intravenous infusion of GIP(1-42) (1 pmol/min/100 g) with a concurrent intraperitoneal glucose load (1 g/kg) significantly reduced circulating blood glucose excursions through stimulation of insulin release. Higher doses of GIP(1-14) and GIP(19-30) (100 pmol/min/100 g) also reduced blood glucose excursions.