To determine the nature of the pancreatic islet cell cholecystokinin (CCK) receptor, we studied CCK receptor binding and biologic activity in isolated rat pancreatic islets. Binding of 70 pM ¹²⁵I-CCK to collagenase-prepared isolated rat pancreatic islets at 24°C was one-half maximal after 5 min and maximal at 60 min. At 60 min, specific binding was 12% of total radioactivity per 100 μg islet protein; nonspecific binding (in the presence of 1 μM CCK 8) was less than 2% of total radioactivity. Unlabeled CCK 33 inhibited labeled hormone binding one-half maximally at 2 nM; Scatchard analysis showed one binding site (K_d, 2.3 ± 0.4 nM; B_max, 8.1 pmol/mg protein). The agonist selectivity of this binding site was: CCK 8 = CCK 33>desulfated-CCK 8>CCK 4. Two CCK antagonists were studied; N-carbobenzoxy-L-tryptophan was more potent than dibutyryl-cGMP. When the effect of CCK on insulin release from the islets was studied, the order of potency of CCK agonists and antagonists on insulin secretion was the same as the order of their ability to inhibit ¹²⁵I-CCK binding. The effect of CCK on insulin secretion was dependent on the glucose concentration in the media. CCK had no effect at 5.6 mM glucose and was fully effective at 11.0 mM glucose. These data, therefore, indicate that: (1) specific binding sites for CCK are present in rat pancreatic beta, cells; and (2) CCK acts in concert with glucose to stimulate insulin secretion.