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-beta-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 = (CH2)n (n = 0-4) reveals that modification of the tetrapeptide reduces affinity to a maximum of micromolar affinity (Boc-Trp-Gly-Asp-Phe-NH2; Ki = 2 X 10(-6) M), whereas in the series when n = 0 and 2 pentamolar affinity is still retained (Boc-Trp-Phe-NH2, Ki = 7 X 10(-5) M; Boc-Trp NH CH2-CH2-CO-Phe-NH2, Ki = 3 X 10(-5) M). Modification of the tetrapeptide CCK-30-33 reduces affinity 1000-fold, whereas di- and tripeptide fragments are identified that reduce affinity only a further 10-fold. This structure-activity relationship establishes a basis to design "peptoid" analogues of CCK that have therapeutic potential.