Although there is no amino acid sequence similarity between maxadilan (Maxa) and pituitary adenylate cyclase activating polypeptide (PACAP), our synthetic Maxa was found to bind PACAP specific receptors (PAC-1 receptors) with a high affinity, but low potency for the accumulation of cAMP in PC12 cells. Competitive binding studies of 125I-PACAP-27 to rat cortical membranes allowed exploration of the structural requirements for this interaction using mini-libraries constructed by solid-phase peptided synthesis, that include disulfide isomers, N-, C- and middle segment deleted peptides and analogs. Maxa as well as PACAP38 inhibited the specific binding of 125I-PACAP-27 with IC50 values of 3.89 and 4.90 nM, respectively. The most potent derivative of our synthetic Maxa-analogs with an IC50 value of 1.99 nM was Maxa[1–23 + 43–61, S-S14–51 Ala1,5] which consists of N- (position 1–23) and C- (position 43–61) terminal linear fragments cross-linked by a disulfide bridge between positions 14 and 51. This peptide did not increase intracellular cAMP, at a concentration of 100 nM, but inhibited cAMP accumulation induced by 1 nM PACAP-27 in PC12 cells, whereas wild Maxa increased intracellular cAMP although it was weaker than PACAP-27. Our data suggest deletion of the middle segment between residues 24–42 affords some derivatives that behave as low affinity antagonists.