3-(Acylamino)-5-phenyl-2H-1,4-benzodiazepines, antagonists of the peptide hormone cholecystokinin (CCK), are described. Developed by reasoned modification of the known anxiolytic benzodiazepines, these compounds provide highly potent, orally effective ligands selective for peripheral (CCK-A) receptors, with binding affinities approaching or equaling that of the natural ligand CCK-8. The distinction between CCK-A receptors on the one hand and CNS (CCK-B), gastrin, and central benzodiazepine receptors on the other is demonstrated by using the structure-activity profiles of the new compounds. Details of the binding of these agents to CCK-A receptors are examined, and the method of development of these compounds is discussed in terms of its relevance to the general problem of drug discovery.