Cholecystokinin acts on

CCK acts on both the pancreas and gall bladder and stimulates the secretion of pancreatic enzymes and bile juice, respectively. 

CCK stimulates the secretion of pancreatic juice that is rich in digestive enzymes in response to chyme. 

It also causes contraction of the wall of the gallbladder, which squeezes stored bile out of the gallbladder into the cystic duct and through the common bile duct. 

In addition, CCK causes relaxation of the sphincter of the hepatopancreatic ampulla (sphincter of Oddi), which allows pancreatic juice and bile to flow into the duodenum.