This study was designed to determine whether cholecystokinin (CCK) plays a physiological role in the inhibition of gastric emptying. Physiological conditions were simulated by giving CCK by continuous intravenous infusion rather than by bolus injection, by using doses known to be distinctly submaximal for pancreatic protein secretion, and for gallbladder contraction, and by releasing endogenous CCK. The rate of gastric emptying was determined in 4 dogs with gastric fistulas by measuring the volume of fluid remaining in the stomach 10 min after instillation of 300 ml of 0.15 M NaCl. Rate of emptying was studied during intravenous infusion of saline (control) and of different doses of 98% pure CCK, commerically available 20% pure CCK, synthetic COOH-terminal octapeptide of CCK (OP-CCK), pentagastrin, and heptadecapeptide gastrin. The effect of endogenously released CCK was studied by measuring the rate of emptying of solutions in which different concentrations of tryptophan replaced equiosmolar amounts of NaCl. The d50's of 20% pure CCK (3 U kg minus-1 hr minus-1) and of OP-CCK (125 ng kg minus-1 hr minus-1) for inhibition of gastric emptying were about the same as their D50's for cholecystokinetic and pancreozyminic actions. By contrast, although both pentagastrin and heptadecapeptide gastrin inhibited gastric emptying, the doses required for this action were much higher than the D50's required for stimulation of gastric acid secretion. The effectiveness of OP-CCK indicates that inhibition of gastric emptying is attributable to CCK itself and not to an impurity in the CCK preparation. We have confirmed this directly by showing that pure CCK is a potent inhibitor of gastric emptying. Tryptophan also inhibited gastric emptying. In other dogs pancreatic protein secretion and gallbladder contraction were shown to be stimulated during the time tryptophan was inhibiting gastric emptying. This evidence supports the view that inhibition of gastric emptying is one of the physiological actions of CCK, but in the case of gastrin it must be regarded as a pharmacological action.