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This study was designed to determine the involvement of cholecystokinin (CCK) in the control of gastric acid secretion in men using loxiglumide, a specific CCK-receptor antagonist. Two groups of healthy subjects (A and B) were used: group A for studies of postprandial gastric secretion and group B for studies with exogenous gastric secretagogues. The cephalic phase activated by modified sham feeding (MSF) in group A subjects increased gastric acid secretion to about 36% of pentagastrin maximum, but treatment with loxiglumide in a standard dose (20 μmol/kg i.v. loading dose plus infusion of 20 μmol/kg/h afterwards) failed to affect this secretion. A 5% peptone meal instilled i.g. (to mimic the gastrointestinal phase) greatly enhanced gastric acid secretion and plasma gastrin concentration, but the addition of loxiglumide in a standard dose resulted in a further increase in both gastric acid and plasma gastrin responses to the peptone meal. Plasma somatostatin response to the peptone meal was significantly reduced by loxiglumide. Infusion of cerulein in gradually increasing doses (15–120 pmol/kg/h) and gas-trin-releasing peptide (GRP) (25–200 pmol/kg/h) resulted in dose-dependent stimulation of gastric acid secretion, reaching about 35 and 25% of the maximum attained with pentagastrin. When loxiglumide was added the acid responses to cerulein and GRP were further increased by two- to threefold, attaining a peak similar to the pentagastrin maximum. Administration of loxiglumide resulted in a significant increase in plasma gastrin response to GRP, whereas plasma somatostatin was not significantly altered by loxiglumide. We conclude that CCK released by a peptone meal or by GRP exerts a potent inhibitory influence on gastric acid secretion and gastrin release in men and that this inhibition involves subtype A CCK/ gastrin CCK receptors.