To elucidate the mechanisms involved in the regulation of CCK release from the small intestine, the effect of neuroactive agents on the release of CCK from the isolated, perfused porcine duodenum has been studied. CCK in the venous effluent was measured by three radio-immunoassays specific for the N-terminus, the tyrosine-o-sulphated region and the C-terminus of CCK-33. Gastrin-releasing peptide (GRP) at concentrations of 10(-10) and 10(-9) M in the arterial line increased the CCK output by about 100 and 500%, respectively, as measured with the assay specific for the sulphated region of CCK. Pilocarpine and physostigmine at concentrations of 10(-5) M reduced the CCK output to 24 and 40% of basal, respectively, and also abolished the GRP-stimulated CCK response. Atropine (10(-6) M) increased CCK output about 100%. The results suggest that both stimulatory and inhibitory nervous impulses may be transmitted to the intestinal CCK cell, mediated by intramural GRP neurons and cholinergic pathways, respectively.