To study the structure-activity relationships of neuromedin U-8 (NMU-8) (H-Tyr-Phe-Leu-Phe-Arg-Pro-Arg-Asn-NH2) and to develop a NMU-8 antagonist, twenty-three NMU-8 analogs substituted with Gly or the corresponding D-amino acid(s) at positions 1-8 were synthesized by solid-phase techniques. On isolated chicken crop preparations, the contractile activity of the synthetic NMU-8 analogs was compared with that of NMU-8 and their antagonistic activity was assayed against NMU-8. The replacement of Phe2, Phe4, Arg5, Pro6, Arg7 or Asn8 with Gly brought about a drastic decrease of the agonistic activities. Substitution of the corresponding D-amino acid residue for Phe2, Phe4, Arg5, Pro6 or Asn8 caused a marked decrease of the agonistic activities, while the replacement of Tyr1 with D-form enhanced the activity. It was further revealed that [D-Pro6]-NMU-8 and [D-Leu3, D-Pro6]-NMU-8 exerted a non-competitive antagonistic activity against NMU-8 with x values of 5.22 +/- 0.12 and 5.34 +/- 0.09, respectively. [D-Phe2, D-Pro6]-NMU-8, [D-Arg5, D-Pro6]-NMU-8 and [D-Pro6, D-Asn8]-NMU-8 showed a very weak antagonism. The results indicated that 1) the side chain of each amino acid at positions 2, 4, 5, 6, 7 and 8 of NMU-8 is of relative importance for the expression of the contractile activity, and 2) [D-Pro6]-NMU-8 and its four analogs acted as an antagonist against NMU-8.