Analysis of the competitive inhibition of 125I-labeled cholecystokinin octapeptide (CCK-8) binding to isolated rat or mouse pancreatic acini showed that in both species CCK-8 interacts with two different affinity sites. A newly synthesized CCK analogue modified at the COOH-terminal phenylalanine residue totally inhibited 125I-CCK binding. This interaction occurred with sites of a single affinity in rat acini but with two different affinity sites in mouse acini. When acini were incubated with increasing concentrations of CCK-8, a biphasic stimulation of amylase release was observed. By use of rat acini, the analogs stimulated amylase release but caused no inhibition at supramaximal concentrations. By contrast, in mouse pancreatic acini, analogues showed a biphasic stimulation of amylase release similar to CCK-8. Both CCK-8 and the analogue stimulated [3H]leucine incorporation into protein at low concentrations in rat pancreatic acini. Higher concentrations of CCK-8 profoundly inhibited [3H]leucine incorporation, whereas the analogue had no inhibitory effect. Moreover, the analogue at higher concentrations blocked the inhibition of [3H]leucine incorporation caused by CCK-8 but did not affect carbamylcholine-induced inhibition. In mouse acini, however, the CCK analogue inhibited [3H]leucine incorporation similar to the effect of CCK-8. The results support the concept that occupancy of distinct affinity sites or states of the CCK receptor is associated with specific biological actions. A model of the CCK receptor is proposed in which two interchangeable affinity states exist. By occupying all the receptors in only one state, the new CCK analogues serve as partial agonists of some and antagonists of other actions of CCK.