The secondary structure of a bradykinin B₁receptor antagonist B-10324 (F5C-Lys-¹-Lys⁰-Arg¹-Pro²-Hyp³-Gly⁴-CpG⁵-Ser⁶-DTic⁷-CpG⁸) was determined by NMR at 800MHz. The conformational data are compared with those obtained previously for two bradykinin B₁ receptor antagonists, namely B-9858 (Lys-¹-Lys⁰-Arg¹-Pro²-Hyp³-Gly⁴-Igl⁵-Ser⁶-DIgl⁷-Oic⁸) and B-10148 (Lys-¹-Lys⁰-Arg¹-Pro²-Hyp³-Gly⁴-Igl⁵-Ser⁶-DF5F⁷-Oic⁸). The abnormal amino acids are: Hyp, trans-4-hydroxyproline; Tic, 1, 2, 3, 4-tetrahydroisoquinoline-3-carboxylic acid; Oic, (2S, 3aS, 7aS)-octahydroindole-2-carboxylic acid; Igl, α(2-indanyl)glycine; F5F, 2,3,4,5,6-pentafluorophenylalanine; CpG, α-cyclopentylglycine. F5C, pentafluorocinnamoyl, is the N-terminal protecting group and is not involved in the peptide secondary structure. B-10324 contains an N-terminal Pro²-CpG⁵ distorted type II β-turn whereas the rest of the peptide is random. A salt bridge is not observed between the carboxylate group at the C-terminal end and the Arg¹ side chain, in contrast to that previously observed for B-9858 and B-10148. The conformations are correlated with the measured B₁ receptor antagonist activities (J.-F. Larrivée, L. Gera, S. Houle, J. Bouthillier, D.R. Bachvarov, J.M. Stewart and F. Marcéau, Br. J. Pharmacol. 131, 885-892 (2000)). The importance of the N-terminal β-turn is highlighted.