CCK-30-33 has been identified as the minimum fragment of CCK with nanomolar affinity for the central CCK receptors, as assayed by displacement of [3H]-Boc-p-alanyl-CCK-30-33 (pentagastrin) in homogenized mouse cerebral cortex. Examination of binding using this assay in the two series Boc-Trp-X-Phe-NH2 when X= Met-Asp (Boc-CCK-30-33), Gly-Asp, Met-Gly, and Gly-Gly and when X=(CH,),(n= 0-4) reveals that modification of the ...