Nociceptin and its receptor (OP(4)) share sequence homologies with the opioid peptide ligand dynorphin A and its receptor OP(2). Cationic residues in the C-terminal sequence of both peptides seem to be required for selective receptor occupation, but the number and the distribution of these basic residues are different and quite critical. Both receptors are presumably activated by the peptides N-terminal sequence (Xaa-Gly Gly-Phe, where Xaa = Phe or Tyr); however, although OP(4) requires Phe(4) as a determinant pharmacophore, OP(2) requires Tyr(1) as do the other opioid receptors. An extensive structure-activity analysis of the N-terminal tetrapeptide has led to conclude that the presence of aromatic residues in position one and four, preferably Phe, as well as the distance between Phe(1) and Phe(4) are extremely critical for occupation and activation of OP(4) in contrast with other opioid receptors (e.g. OP(1), OP(3), OP(2)). Modification of distance between the side chains of Phe(1) and Phe(4) (as obtained with Nphe(1) substitution in both NC and NC(1-13)-NH(2)) and/or conformational orientation of Phe(1) (as in Phe(1)psi(CH(2)-NH)-Gly(2)) has brought to discovery of pure antagonist ([Nphe(1)]-NC(1-13)-NH(2)) and a partial agonist ([Phe(1) psi(CH(2)-NH)-Gly(2)]-NC(1-13)-NH(2)), which have allowed us to characterize and classify the OP(4) receptor in several species. Thus, although antagonist activities at the OP(4) receptor are obtained by chemical modification of Phe(1)-Gly(2) peptide bond or by a shift of Phe(1) side chain of NC peptides, antagonism at the OP(2) receptor requires the diallylation of the N-terminal amino function, for instance, of dynorphin A. These considerations support the interpretation that the two systems nociceptin/OP(4) and dynorphin A/OP(2) are distinct pharmacological entities that differs in both their active sites (Tyr(1) for Dyn A and Phe(4) for NC) and the number and position of cationic residues in the C-terminal portions of the molecules. The chemical features of novel OP(4) receptor ligands either pseudopeptides obtained by combinatorial library screening or molecules of nonpeptide structure are reported and discussed in comparison with NC and NC related peptides.