(Bu)2cGMP is known to act as a specific competitive inhibitor for gastrin and cholecystokinin (CCK) peptides. We have examined the effects of (Bu)2cGMP on CCK octapeptide (CCK-8) stimulation of insulin release in the isolated perfused pancreas and compared them with those on protein output. Addition of (Bu)2cGMP after a 20-min perfusion with 100 pM CCK-8 resulted in two distinctly different phases of insulin suppression. There was a sharp initial decline in insulin release for 3 min, followed by transient recovery toward the control level for 5 min, and then a small decline until termination of (Bu)2cGMP infusion. (Bu)2cGMP produced a concentration-dependent inhibition of both phases of insulin decrement. (Bu)2cGMP also produced a concentration-dependent inhibition of protein output. Addition of 1 mM (Bu)2cGMP rapidly and completely abolished CCK-8-stimulated protein output. Since CCK is released by meal intake and exogenous CCK stimulates insulin release and augments glucose-induced insulin release, it is possible that endogenous CCK plays an important role in the enteroinsular axis. The present findings of blockade of CCK-8-induced insulin release by selective antagonist of the action of CCK provide evidence for CCK as a mediator in the enteroinsular axis.