The structural requirements for VIP interaction with receptors on synaptosomes from rat cerebral cortex was investigated by the ability of VIP and VIP fragments, secretin analogues and fragments, peptides of the VIP/secretin family and several other regulatory peptides to inhibit specific 125I-VIP binding. Only large VIP fragments interacted with the VIP receptors with potencies relative to VIP ranging from 0.9-0.006%. The rank order of inhibition was: VIP 7-27 greater than VIP 11-28 greater than VIP 1-22-NH2 greater than VIP 16-28. Shorter fragments: VIP 18-28; VIP 18-28-NH2; VIP 19-28; VIP 21-28; VIP 22-28; VIP 1-18; VIP 1-18-NH2; VIP 1-10-NH2; VIP 1-6; VIP 16-20 and VIP 16-19 had no effect. Secretin fragments and analogues inhibited 125I-VIP binding with potencies of 2.2-0.01% relative to VIP in the order; secretin greater than (Ala4, Val5) secretin greater than (D-Ala4) secretin greater than (D-Phe6) secretin greater than secretin 5-27 greater than secretin 14-27. Other peptides of the VIP/secretin family inhibited 125I-VIP binding with potencies of 200-1%; avian VIP greater than porcine VIP greater than PHI = secretin greater than human GRF, whereas glucagon and GIP showed no inhibition. Among twenty-five other regulatory peptides only avian PP and somatostatin were inhibitors with relative potencies of 0.02% and 0.03%, respectively. In conclusion it may be emphasized that the intact VIP molecule is essential for VIP interaction with its receptors in the rat brain cortex.(ABSTRACT TRUNCATED AT 250 WORDS)