Two pentapeptides with opiate activity, [Met5] enkephalin and [Leu5] enkephalin, were studied by means of PMR, CMR, UV and CD spectroscopies in different solvents and at different concentrations. The primary result which we report is the demonstration of a concentration dependence. Spectral properties which are characteristically used to evaluate conformation are shown to differ at different concentrations. This provides an explanation for conflicting results of previous studies. Two conformational states of enkephalins which are consistent with the data are considered: i) A monomeric form, containing a β-turn with Gly3 and Phe4 at the corners, a 7-atom H-bond and the folding of the Tyr1 aromatic side chain over the molecule stabilized by an interaction of its OH proton with the Gly3 CO. ii) An associated form with an antiparallel cross-β-structure stabilized by four intermolecular H-bonds and with a "head to tail" interaction. © 1977.