In this study, the techniques of computational chemistry were used to probe the origin of the differing pharmacological profiles found as a result of two simple modifications of the enkephalin analog Tyr–DAla–Gly–Phe–MetNH2: the presence or absence of N-methylation of Phe4 and of the Met5 residue. Although all four analogs have high μ-receptor affinity, their analgesic activity varies by a factor of 3000. Thus, they should share common determinants of μ-receptor recognition while differing in the ability to activate the receptor. To identify and characterize these determinants, a two-step procedure was used. In the first step, the energy conformational profile of each peptide was obtained. The strategy used involved the iterative calculation of molecular dynamics trajectories at high and low temperatures, coupled to energy minimizations, allowing a through sampling of conformational space. In the second step, low-energy conformers of the four peptides were examined for the extent to which they fulfilled the requirements for μ-receptor recognition recently developed for nonpeptide analogs. In these studies, the amine nitrogen, a second proton-accepting moiety, and an aromatic ring in a specific geometric arrangement were proposed as the minimum components of a μ-pharmacophore for recognition. For all four analogs, a unique low-energy conformer was found that contained these three recognition moieties in a geometric arrangement to interact with the same target binding site residues as in the nonpeptide analogs. These results are consistent with the finding of high affinity for all four peptides and provide common determinants of recognition of the μ-receptor by peptides and nonpeptides. When the four peptides were overlapped so that they could each interact with these three common recognition sites, the Phe4 aromatic side chain was found to be a possible modulator of activation. For the parent pentapeptide, Tyr–DAla–Gly–Phe–Met, with the lowest activity, there was poor overlap of the Phe4 aromatic ring with the same ring in the other three analogs. These results implicate the Phe4 ring in peptide activation of the μ-receptor. © 1993 John Wiley & Sons, Inc.