Cholecystokinin (CCK) is a gut hormone that regulates pancreatic endocrine functions via CCK<sub>A</sub> receptors. CCK<sub>4</sub> (Trp-Met-Asp-Phe-NH<sub>2</sub>) has an insulinotropic effect, but is 1,000-fold less potent than CCK<sub>8</sub> in rodents. The in vitro potencies with respect to binding, the biological effects and the selectivity of newly synthesized CCK<sub>4</sub> analogs constructed by computer modelling experiments were investigated in vitro in rat pancreas and brain, INS-1 cells, and guinea pig ileum. Exchanging various amino acids, e.g. Met by either Pro or Nle, and modifying Phe by adding various substituents in different positions led to compounds which were more effective as insulin secretagogues than CCK<sub>4</sub> itself and even show insulinotropic effects comparable with those of CCK<sub>8</sub> (e.g. compounds M1 and M2 being substituted at Phe). Some compounds which possess electron withdrawing groups on the C-terminal Phe and possess a Pro instead of a Met were especially effective. The CCK<sub>A</sub> receptor antagonist L-364,718, but not by the CCK<sub>B</sub> receptor antagonist L-365,260, inhibited the insulinotropic effects. The synthetic CCK<sub>4</sub> compounds were not selective for the endocrine pancreas: e.g. M1 and M2 had binding activity with respect to rat brain homogenates but no activity with respect to contraction of the guinea pig ileum. The data indicate that some of the newly synthesized CCK tetrapeptides exhibit a high affinity for the CCK receptor of β-cells and have an insulinotropic effect much higher than CCK<sub>4</sub>.