The conformational space of the potent mu-selective opioids [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAGO) and [D-Ala2,MePhe4,Met(O)-ol5]enkephalin (FK 33-824) has been analyzed by 1H-NMR spectroscopy and theoretical calculations involving systematic conformational searching and energy minimizations. A cis-trans equilibrium of the Gly3-MePhe4 amide bond is induced by the N-methyl group, and the more energetically favoured trans isomer is proposed as the biologically relevant form. A compact interaction between the side chains of Tyr1 and D-Ala2 was demonstrated by NOE and ROE effects in both peptides in D2O and DMSO-d6, further supported by shielding of the D-Ala2 methyl protons in both solvents. Analysis of coupling constants, NOE and ROE data indicated significant restriction of the conformational freedom of the MePhe4 side-chain for both peptides in the two solvents. The NMR results and theoretical calculations point towards folded low energy conformations characterized by a beta II-type turn around Gly3-MePhe4. For the trans isomer, a Tyr1-MePhe4 phenyl ring separation between 8.5 and 12.5 A was accompanied by proximity between the D-Ala2 side chain and the C-terminal in low energy conformations. The results are in good agreement with available data on related active enkephalins. The conformational effects induced by simultaneous incorporation of D-Ala2 and MePhe4 in enkephalins is discussed in the light of the enhanced mu-opioid receptor selectivity and activity of these peptides.