The conformational properties of somatostatin, a tetradecapeptide hormone with a disulfide bond joining cysteine residues at positions 3 and 14, and of the reduced dithiol form of somatostatin, the two somatostatin-glutathione single mixed disulfides and somatostatin-glutathione double mixed disulfide were investigated in aqueous solution by one-and two-dimensional 'H NMR. Resonances were assigned using connectivities derived from 2D TOCSY and ROESY spectra. 'H-'H ROES together with the temperature coefficients for the chemical shifts of amide NH resonances for the native disulfide form of somatostatin are consistent with previous conclusions that somatostatin exists as an equilibrium of several rapidly interconverting conformations ; however, the ROE results provide more information about the conformations than was obtained from previous studies. In particular, CaHrNH,+, ROEs together with NHrNH,+, ROEs identify specific p-turns in the family of low-energy conformations. The NMR results indicate that reduced somatostatin and the somatostatin-glutathione mixed disulfides have some of the same pturn conformational features as cyclic somatostatin, but that the populations of the conformations having the fi-turns are less for the acyclic peptides.