We have examined the effect of cholecystokinin octapeptide (CCK-8) on the ability of carbamylcholine (carbachol) to inhibit binding of 3H-N-methylscopolamine (3H-NMS) to muscarinic receptors on dispersed acini from rat pancreas. Carbachol bound to muscarinic receptors on rat dispersed pancreatic acini with two different Kd values. EC50 value for the stimulation of amylase secretion by carbachol was well compatible to the calculated Kd value of high affinity binding site of carbachol, suggesting that high affinity binding sites were mainly involved in the secretory response. Simultaneous addition of CCK-8 decreased the ability of carbachol to inhibit binding of 3H-NMS to acini due to an apparent disappearance of high affinity binding site of carbachol. The effective range of concentrations of CCK-8 for such a receptor regulation, as well as the time course of CCK-8 effect, were in good agreement with those for the restriction of carbachol-induced amylase secretion by CCK-8. These findings suggest that the restriction of carbachol-stimulated amylase secretion by CCK-8 is mainly a result of the disappearance of high affinity binding site of muscarinic receptor as a result of CCK-8 administration.