The backbone conformation of the 27-residue polypeptide hormone secretin has been investigated using nuclear magnetic resonance spectroscopy and restrained molecular dynamics calculations under conditions where it adopts a fully ordered structure (40% v/v trifluoroethanol). The basis for the restrained molecular dynamics calculations consists of 52 nuclear-Overhauser-enhancement-derived interproton distance restraints involving the NH, C alpha H and C beta H protons. It is shown that convergence to similar extended structures is achieved starting from four different initial structures, namely an alpha helix, a mixed alpha/beta structure, a beta strand and a polyproline helix. The converged structures are made up of short N- and C-terminal strand-like regions and a central region comprising two irregular helices connected by a 'half-turn'.