We have determined the three-dimensional structure of [C-13,N-15]-labeled Leu-enkephalin (Tyr-Gly-Gly-Phe-Leu) dihydrate (crystallized from aqueous methanol) on the basis of six sets of accurately determined C-13 ...N-15 interatomic distances by rotational echo double resonance (REDOR) and some additional constraints from C-13 chemical shifts. This compound has not yet been refined by X-ray diffraction. Six kinds of [C-13,N-15]-doubly-labeled samples, in which the doubly-labeled positions are four-bonds apart (four samples) and five-bonds apart (two samples), were chemically synthesized. These labeled peptides (100%) and an isotopically diluted one with unlabeled samples (60% or 30%) were crystallized from aqueous methanol solution. C-13 or N-15 chemical shifts were carefully evaluated prior to and after every REDOR experiment in order to check that the crystalline polymorphs under consideration were not modified either by loss of or by freezing of motion of solvent molecules in the crystals. Accurate and precise interatomic distances (+/-0.10 Angstrom) were obtained from REDOR factors of infinite dilution, which were extrapolated from the data of 100%, and 30% isotopically diluted samples to eliminate dipolar contributions from the labeled nuclei of neighboring molecules in the crystals. These distance data were converted to a possible set of local torsion angles (phi(i) and psi(i)) in a peptide unit of the respective amino acid residue of interest using standard bond lengths and angles in a sequential manner. It turned out that a unique set of the torsion angles corresponding to the most appropriate three-dimensional structure was determined with reference to some additional constraints from the conformation-dependent displacements of C-13 chemical shifts of certain peptide units. The three-dimensional structure thus obtained was finally subject to a calculation for energy minimization in order to ensure that the conformation obtained was at least at one of local minima. Finally, the biological consequence of the peptide structure thus determined is discussed.