The binding to [125I]PACAP27 and adenylate cyclase activity have been investigated using rat brain membranes with substituted analogues of PACAP and VIP, including their hybrid peptides. Binding of [125I]PACAP27 was rapid, specific and reversible. Scatchard analysis revealed a single class of binding site, with a Kd = 457 +/- 117 pM, and a Bmax = 2.63 +/- 0.24 pmol.mg protein-1. Hybrids of PACAP, in which specific residues were substituted with the corresponding residues of VIP, and vice versa, as well as related analogues, were then tested for binding and adenylate cyclase activity. The results showed that N-terminal residues were important for recognition. In particular, multiple substituted analogues of PACAP by VIP, and vice versa, demonstrated that positions 4, 5 and 9 play a dominant role in the recognition of PACAP Type I receptor in rat brain membranes and account for the differences observed between PACAP and VIP. Substitutions in the C-terminal region at positions 24, 25 and 26 are not crucial for recognition specificity. PACAP-analogues provide evidence that positions 1 and 6 are essential for receptor recognition. The flexibility at position 21 also appears to play a role as substitution with Ala or Phe is tolerated, while Pro shows a significant loss both in binding affinity and adenylate cyclase activity.