A series of analogues of nociceptin, Noc(1-13)NH(2) (an agonist at the ORL1 receptor) was synthesized with following modifications: (1) N-terminal extension with Arg(0); (2) replacement of Gly(3) by basic or polar amino acids-Arg, Asn, Lys(For) or deletion; (3) exchange of Phe(1) or Phe(4) by Phe(NO(2)); (4) substitution of Ser(10) with D-Ser, Pro, D-Pro. The analogs were synthesized by solid-phase methodology using Fmoc-amino acid pentafluorophenyl esters. The affinity for the ORL1 and for the kappa, micro and delta-opioid receptors was investigated by radioligand binding assay and bioactivity by a mouse vas deferens (MVD) assay. The addition of the amino acid residue Arg to the N-terminal enhances the opioid receptor affinity of Noc(1-13)NH(2) while retaining ORL1 receptor affinity at a moderate level. The replacement of Gly in position 3 by the basic or polar amino acids-Arg, Asn, Lys(For) or its deletion led to inactive analogues. The replacement of Ser in position 10 by its D-isomer, Pro and D-Pro resulted in a series of analogues with the following order of activity: Ser(10)>D-Ser(10)>Pro(10)>D-Pro(10). In [D-Ser(10)]Noc(1-13)NH(2), introduction of an additional Phe(NO(2))(4) led to a >60-fold increase of ORL1 affinity, completely attenuating the loss of affinity brought about by Ser(10). In other analogues, introduction of Phe(NO(2))(4) did not change the magnitude of ORL1 binding significantly. Generally, while modifications in position 3 frequently led to a loss of most or all bioactivity, modifications in position 0 (Arg(0)) or 4 (Phe(NO(2))(4)) and 10 (D-Ser(10), Pro(10)) are tolerated.