Both the aqueous and the lipid-induced structure of a representative and widely studied tachykinin, substance P, has been investigated by two-dimensional proton nuclear magnetic resonance (2D 'H-NMR) spectroscopy and distance geometry calculations. Unambiguous NMR assignments of protons have been made with the aid of correlation spectroscopy (COSY and TOCSY) experiments and Overhauser enhancement spectroscopy (ROESY and NOESY) experiments. The NMR data obtained were utilized in a distance geometry algorithm to generate a family of structures which were further refined using restrained energy minimization. These data show that, while in water substance P appears to favour an extended chain conformation, in the presence of perdeuterated dodecylphosphocholine (DPC) micelles as membrane model system an amphiphilic helical conformation is induced in the mid-region (Q5-Q8) of substance P. The conformation adopted by substance P in the presence of DPC micelles yields a structural motif typical of neurokinin-1 selective ligands, as proposed by Convert and coworkers (O. Convert et al, Neuropeptides 19, 259–270 (1991)).