Salmon calcitonin, a 32-residue peptide with a 1–7 disulfide bridge, was synthesized by standard solid-phase techniques, and studied by CD and two-dimensional NMR experiments. The peptide was dissolved in pure trifluoroethanol (TFE) and in aqueous solutions containing various amounts of TFE. CD studies in pure TFE indicated the presence of an α-helical structure comprising 40% of the constituent amino acids. This was fully confirmed by nmr. A detailed analysis was performed with the peptide in a 9 : 1 deuterated TFE/H2O mixture. A total of 365 nuclear Overhauser enhancements (154 intraresidual, 112 sequential and 99 long range) were compiled from the nuclear Overhauser enhancement spectroscopy spectra and used in the distance geometry calculations. The core of the peptide between residues 8 and 22 assumes an α-helix like structure. The Cys 1–Cys 7 ring is well defined and in close association with the helix, while the C-terminal decapeptide folds back toward the core, forming a loose loop.