The contractile action of cholecystokinin (CCK) on smooth muscle of the gut is either direct (gallbladder and gastric fundus) or both direct and neurally mediated (small intestine). These regional differences were used to characterize pharmacologically CCK receptors on smooth muscle cells and neurons of the gastric fundus, gallbladder, and ileum of the guinea pig. In circular and longitudinal ileal smooth muscle, tetrodotoxin was used to separate direct from neurally mediated contractile effects. Cholecystokinin receptors on smooth muscle cells were found in all locations. The muscle cells displayed a decreasing order of sensitivity to the C-terminal octapeptide of cholecystokinin as expressed in the median doses, and to the selective cholecystokinin antagonist, proglumide, as expressed in the inhibitory dissociation constants. The median doses of the octapeptide of cholecystokinin ranged from 5.5 nM in gallbladder muscle to 185 nM in circular ileal muscle; the corresponding inhibitory dissociation constants of proglumide ranged from 180 to 437 microM [corrected]. Cholecystokinin receptors on cholinergic neurons were confined to circular and longitudinal ileal muscle; the neurons were 80-300 times more sensitive to the octapeptide of cholecystokinin (D50's 0.5 and 2.3 nM) than the corresponding muscle cells, and 19-21 times more sensitive to proglumide (inhibitory dissociation constants, 20 microM [corrected]). The results provide clear evidence of cellular heterogeneity of cholecystokinin receptors (i.e., difference in sensitivity between muscle cells and neurons from the same location) as well as regional heterogeneity (i.e., difference in sensitivity between muscle cells from various locations).