The purpose of this study were 1) to quantify the lower esophageal sphincter (LES) response to intravenous cholecystokinin (CCK) in both man and the opossum in vivo, 2) to characterize the interaction of CCK and gastrin on circular muscle of the LES, and 3) to determine the site of action of CCK on LES muscle. In both man and the opossum LES pressure was decreased significantly by either constant intravenous infusion or bolus injection of CCK. In vitro dose-response curves to gastrin I, CCK, and the octapeptide of CCK (OP) demonstrated that both CCK and OP were partial agonists on the LES muscle. Both CCK and OP contract LES muscle at lower threshold doses, but give smaller maximum responses than gastrin I. The maximum response of LES muscle to CCK was antagonized only by atropine and tetrodotoxin, but not by other antagonists, suggesting that CCK contracts LES muscle by acetylcholine release. In vitro studies on LES muscle showed that CCK selectively antagonized the effect of gastrin I, but not other agonists. These studied suggest that CCK reduces LES pressure in vivo by inhibition of the endogenous gastrin effect.