The biochemical characteristics of specific binding sites for gastrin releasing peptide were examined in a preparation of the mucosa and submucosa of the porcine jejunum. Iodinated gastrin releasing peptide bound to sites in this preparation with an affinity that was similar to its potency in stimulating anion transport in the epithelium. The relative order of potency of peptide analogues of gastrin releasing peptide in contracting longitudinal smooth muscle was similar to their order of potency in altering anion transport across the mucosa. Autoradiographic studies showed that specific binding sites for [125I]gastrin releasing peptide were associated with the epithelium of the mucosa as well as the myenteric plexus. Gastrin releasing peptide-like immunoreactive axons paralleled the distribution of specific binding sites for the peptide. These data support a physiological role for gastrin releasing peptide in the control of intestinal ion secretion and motor function.