The effects of cholecystokinin and cholecystokinin-octapeptide on the canine cystic duct have been studied. The resistance to flow through the duct in vivo, assessed by measuring the flow rate of saline that was perfused through the duct at constant pressure, was increased by cholecystokinin in a dose-dependent manner. Cholecystokinin also produced the well-recognized, dose-dependent elevations of gallbladder intraluminal pressure. Comparison of dose-response curves prepared in each animal indicated that the sensitivity of the cystic duct was significantly lower than that of the gallbladder. Isolated strips of cystic duct and gallbladder in vitro were contracted by cholecystokinin or cholecystokinin-octapeptide; again, cystic duct muscle displayed a lower sensitivity than gallbladder to the contractile effects of these agents. It is not known whether cholecystokinin can affect cystic duct resistance under physiologic conditions.