The canine lower esophageal sphincter (LES) was evaluated as a model for investigation of the electrophysiology of nonadrenergic noncholinergic neurotransmission. The LES contained a dense collection of gap junctions and a moderate number of nerves located at the periphery of muscle bundles, which are often associated with interstitial cells of Cajal. These cells make gap junctions with one another and with smooth muscle cells. The nerve varicosities contained many large dense cored vesicles. In vitro the LES produced spontaneous active tone and relaxed to field stimulation that was blocked by tetrodotoxin (TTX). Increase of stimulus duration or frequency produced a partially TTX-insensitive sphincter relaxation. Basal tone and nerve-mediated relaxation were unaffected by cholinergic, histaminergic, or adrenergic antagonists. Acetylcholine, carbachol, or histamine caused TTX-insensitive contractions mediated by M2- and H1-receptors, respectively. Vasoactive intestinal peptide (10(-6) M) and isoproterenol (10(-6) M) relaxed the LES to the same extent as field stimulation. In the double sucrose gap this tissue regularly produced inhibitory junction potentials when field stimulated. The canine LES has appropriate morphological and pharmacological properties for the electrophysiological study of nonadrenergic noncholinergic neural responses, their mechanisms, and mediators.