The binding of cholecystokinin (CCK) to its receptors on guinea pig gastric chief cell membranes were characterized by the use of 125I-CCK-octapeptide (CCK8). At 30 degrees C optimal binding was obtained at acidic pH in the presence of Mg2+, while Na+ reduced the binding. In contrast to reports on pancreatic and brain CCK receptors, scatchard analysis of CCK binding to chief cell membranes revealed two classes of binding sites. Whereas, in the presence of a non-hydrolyzable GTP analog, GTP gamma S, only a low affinity site of CCK binding was observed. Chief cell receptors recognized CCK analogs, with an order of potency of: CCK8 greater than gastrin-I greater than CCK4. Although all CCK receptor antagonists tested (dibutyryl cyclic GMP, L-364718 and CR1409) inhibited labeled CCK binding to chief cell membranes, the relative potencies of these antagonists in terms of inhibiting labeled CCK binding were different from those observed in either pancreatic membranes or brain membranes. The results indicate, therefore, that on gastric chief cell membranes there exist specific CCK receptors, which are coupled to G protein. Furthermore, chief cell CCK receptors may be distinct from pancreatic or brain type CCK receptors.