Analogues of the opioid peptides [D-Phe(3)]morphiceptin (H-Tyr-Pro-D-Phe-Pro-NH2) and endomorphin-2 (H-Tyr-Pro-Phe-Phe-NH2) containing the pseudoproline (Psi Pro) (4R)-thiazolidine-4-carboxylic acid (CyS[Psi (R1,R2) pro]) or (4S)-oxazolidine-4-carboxylic acid (Ser[Psi (R1,R2)pro]) in place of Pro(2) were synthesized. The pseudoproline ring in these compounds was either unsubstituted (R-1, R-2 = H) or dimethylated (R-1, R-2 = CH3) at the 2-C position. 2-C-dimethylated pseudoprolines are known to be quantitative or nearly quantitative inducers of the Cis conformation around the Xaa(i-1)-Xaa(i)[Psi (CH3,CH3)pro] imide bond. All dihydropseudoproline-containing analogues (R1, R2 = H) showed goody opioid agonist potency in the guinea pig ileum (GPI) assay, high mu receptor binding affinity in the rat brain membrane binding assay, and, like their parent peptides, excellent mu receptor binding selectivity. H-1 NMR spectroscopic analysis of the Cys[Psi (H,H)pro](2)- and Ser[Psi (H,H)pro](2)-containing analogues in DMSO-d(6) revealed that they existed in a conformational equilibrium around the Tyr-Xaa[Psi (H,H)pro] peptide bond with cis/trans ratios of 40:60 and 45:55, respectively. The dimethylated thiazolidine- and oxazolidine-containing [D-Phe3]morphiceptin- and endomorphin-2 analogues (R-1, R-2 = CH3) all retained full mu agonist potency in the GPI assay and displayed mu receptor binding affinities in the nanomolar range and high mu receptor selectivity. As expected, no conformers of the latter analogues with a trans conformation around the Tyr-Xaa[Psi (CH3,CH3)pro] imide bond were detected by H-1 NMR spectral analysis, indicating that in these compounds the cis conformation is highly predominant (> 98%). These results represent the most direct evidence obtained so far to indicate that morphiceptin and endomorphin-2 have the cis conformation around the Tyr-Pro peptide bond in their bioactive conformations.