Guanylin and uroguanylin are peptides that activate receptor guanylate cyclases (GCs) and elicit increased intestinal secretion. Bacteria that cause traveler's diarrhea produce heat-stable toxins (STs) that mimic this action. Investigation of the distribution and identity of receptor GCs in the gastrointestinal tract of rats revealed that receptors were localized to epithelial cells in stomach and intestine. Clusters of cells in gastric mucosa and enterocytes lining the intestine exhibited specific binding of 125I-labeled ST. Ligated loops of stomach and intestine treated with intraluminal ST had significant increases in guanosine 3',5'-cyclic monophosphate (cGMP), with duodenum exhibiting the greatest response. Expression of guanylate cyclase C (GCC) mRNA and a truncated, GCC-like mRNA was found in both stomach and intestine. Both mRNAs were isolated as cDNAs encoding the GC catalytic domain. The 0.9-kilobase (kb) cDNA is 99.8% identical to GCC, whereas the truncated, 0.75-kb GCC-like cDNA has a 159-nucleotide deletion and is 96.6% identical to GCC at the protein level. Uroguanylin and guanylin mRNAs were detected in stomach and intestine. Uroguanylin mRNA was most abundant in small intestine, whereas guanylin mRNA was highest in large intestine. Thus the stomach and intestine are targets for regulation of transport by guanylin and uroguanylin via cGMP.