The effect of morphine on the cat choledochoduodenal junction was studied by means of differentin vivoandin vitromethods.In vivo:Morphine (1‐100 μg/kg) induced increased duodenal activity and increased resistance to flow through the sphincter of Oddi when given intravenously. Atropine (5 mg/kg intravenously) counteracted the effect of 1 μg/kg morphine intravenously on the choledochoduodenal tract, but was ineffective in preventing the spasm caused by slightly increased doses of morphine. Effective α‐adrenoceptor blockade with dibenzyline (2 mg/kg) did not affect the action of morphine. Morphine (0.001–0.02 μg/kg) consistently produced increased duodenal activity and increased passage pressure through the sphincter when administered i.a. in the pancreatic duodenal artery.In vitro:The isolated sphincter and the isolated duodenum were insensitive to morphine in concentrations up to 80 μg/ml in the bath fluid.In vivomorphine is active locally in the cat choledochoduodenal tract but inactive when added to the bath fluid of the isolated sphincter of Oddi or duodenum. The results support the view that morphine exerts its spasmogenic effect on the sphincter region by the release of some endogenous contracting agent. This agent does not belong to the cholinergic or adrenergic nervous system but may be released from the vascular compartment b