The interaction of the peptide hormone bradykinin (Arg¹-Pro²-Pro³-Gly⁴-Phe⁵-Ser⁶-Pro⁷-Phe⁸-Arg⁹, BK) and of two analogues containing the paramagnetic amino acid TOAC with model membranes was investigated by steady state fluorescence spectroscopy. TOAC was inserted before Arg¹ (TOAC⁰-BK) or substituting Pro³ (TOAC³-BK). Fluorescence spectra of Phe residues (l_exc = 258 nm) were obtained in aqueous solution and in the presence of zwitterionic (N-hexadecyl-N,N-dimethyl-3-ammonium-1-propane sulfonate (HPS) and 1-palmitoyl-2-hydroxy-phosphatidylcholine (lysoPC)) or negatively charged (sodium dodecyl sulfate (SDS) and 1-palmitoyl-2-hydroxy-phosphatidylglycerol (lysoPG)) detergent micelles and phospholipid bilayers (1-palmitoyl-2-oleoyl  phosphatidylcholine (POPC) and 1-palmitoyl-2-oleoyl phosphatidylglycerol (POPG)). The insertion of paramagnetic TOAC led to intramolecular fluorescence quenching, the analogue at position 3 being more effective due to the closer proximity to Phe residues. Variable pH studies showed that the peptide-membrane interaction caused pK changes of ionizable residues, probably as a consequence of conformational changes and of binding to the less polar environment. Titrations with increasing membrane concentrations allowed the calculation of apparent binding constants (K_b). Our results indicate that, because of their cationic nature, the peptides bound to the anionic model membranes studied. The K_b values indicated that the binding to negatively model membranes followed the order: BK > TOAC⁰-BK > TOAC³-BK. The  lower extent of TOAC³-BK binding probably reflects the effect of the TOAC-imposed bend on the peptide affinity for the membrane. The data show that fluorescence studies can provide relevant information regarding peptide-membrane interaction. Binding to the lipid bilayer has been proposed as preceding the binding of peptide hormones to their membrane-embedded receptors. Thus, the physico-chemical and conformational aspects of peptide-model membrane interactions could play a fundamental role in signal transduction.