By using 13C enrichment in [Leu5]-enkephalin, it was possible to improve the assignment of carbonyl resonances in the NMR spectrum and to remove some of the ambiguities in the derived .vphi. and .chi. dihedral angles, thereby providing the information about the conformation of this molecule in solution. The combined use of 13C and H NMR experiments leads to the conclusion that [Leu5]-enkephalin contains a type I.beta. bend at residues Gly3-Phe4 in dimethyl-d6 sulfoxide (Me2SO-d6) solution. The side chains of Tyr1, Phe4 and Leu5 exist predominantly in 1 conformation (tg, trans gauche minus) in this solvent. A comparison is made between the conformation found in Me2SO-d6 and those determined by X-ray diffraction and conformational energy calculations.