We describe the structure of a small cyclic peptide Cys-Leu-Pro-Arg-Glu-Pro-Gly-Leu-Cys containing two non-vicinal prolines. A previous HPLC analysis showed two well-resolved peaks at low temperature, and the hypothesis of slow conformational equilibrium due to steric constraints induced by cyclization was presented (Gesquiere, J-C,; Diesis, E.; Cung, M. T.; Tartar, A. J. Chromatogr. 1989, 478, 121). Our NMR study shows that at least three conformations of the peptide are present in solution : a major form which is the all-trans conformer and two minor forms where the peptide bonds Leu-Pro or Glu-Pro are in the cis conformation. The first HPLC peak was shown to contain the trans-trans and trans-cis forms of the peptide whereas the second HPLC peak contains the cis-trans form, The combined use of HPLC and NMR techniques led to the kinetic parameters characterizing the isomerization. The solution structure of the trans-trans peptide was determined by NMR spectroscopy and showed a predominant structural role for the proline in position three and for the cysteine bridge, Reduction of the disulfide bridge leads to a peptide where the cis and trans forms of both proline residues are still observable by NMR but where cis/trans isomerization of Pro3 is too fast to be detected on the HPLC time scale.