Molecular dynamics simulations were performed on the potent and slightly μ‐receptor selective cyclic dermorphin analog as well as on analogs containing a conformationally restricted phenylalanine derivative in place of Phe in the 3 position of the peptide sequence. Peptides studied included the potent and highly μ‐selective analogs (Aic = 2‐aminoindan‐2‐carboxylic acid), (Atc = 2‐aminotetralin‐2‐carboxylic acid) and and the weakly active analog (Tic = tetrahydroisoquinoline‐3‐carboxylic acid). Four different starting conformations were chosen for each peptide, and after equilibration each simulation was allowed to proceed for 100 picoseconds at 600°K. The 14‐membered ring structures in the Phe‐, Aic‐, l‐ and d‐Atc‐containing analogs showed moderate structural flexibility, while the peptide ring in the Tic‐containing analog was more rigid. As theoretically predicted, the φ3 and Ψ3 angles of the Aic‐, l‐ and d‐Atc‐containing analogs were limited to values of either about +50° or ‐50° during almost the entire period of the simulations. In the Tic‐containing analog the φ3 and Ψ33 angles were 0° and 90°, respectively, and did not change for the entire duration of the simulation. The side chains of the constrained amino acids showed limited movement, but transitions between the allowed conformations did occur on the time scale of the simulations. One interesting aspect of the 5‐membered ring of the Aic side chain was that it underwent a greater number of conformational transitions than the 6‐membered rings contained in Atc and Tic, but covered a smaller volume of conformational space. Thus, the relative flexibility of these constrained amino acids was Phe > Aic > l‐ and d‐Atc = Tic, but the relative volume of conformational space visited by these same residues was Phe > l‐ and d‐Atc = Tic > Aic. In all compounds studied the exocyclic Tyr1 residue had the greatest flexibility and covered a large volume of conformational space.