The conformational study on N-acetyl-N'-methylamides of oxazolidine and thiazolidine residues (Ac-OxaNHMe and Ac-Thz-NHMe) is carried out using ab initio HF and density functional B3LYP methods with the self-consistent reaction field method to explore the effects of the replacement of the (CH2)-H-y group in the prolyl ring by oxygen or sulfur atoms on the conformational preferences and prolyl cis-trans isomerization in the gas phase and in solution (chloroform and water). As the solvent polarity increases, the conformations C with the C-7 intramolecular hydrogen bonds become depopulated, the PPII- or PPI-like conformations F become more populated, and the cis populations increase for both Oxa and Thz dipeptides, as found for the Pro dipeptide, although the populations of backbone conformations and puckerings are different in pseudoproline and proline dipeptides. As the increase of solvent polarity, the populations of the trans/up conformations decrease for Oxa and Thz dipeptides, but they increase for the Pro dipeptide. It is found that the cis-trans isomerization proceeds through the anticlockwise rotation with omega' approximate to -60 degrees about the oxazolidyl peptide bond and the clockwise rotation with omega' +120 degrees about the thiazolidyl peptide bond in the gas phase and in solution, whereas the clockwise rotation is preferred for the prolyl peptide bond. The pertinent distance d(N center dot center dot center dot - H-N-NHMe) and the pyramidality of the prolyl nitrogen can describe the role of this hydrogen bond in stabilizing the transition state structure but the lower rotational barriers for Oxa and Thz dipeptides than those for the Pro dipeptide, which is observed from experiments, cannot be rationalized. The calculated cis populations and rotational barriers to the cis-trans isomerization