The conformational preferences of PIDOTIMOD, namely 3-(5-oxo-L-propyl)-L-thiazolidin-4-carboxyl acid, a synthetic dipeptide showing a beneficial influence on some immune functions both in mammalians and in humans, have been searched by varying two of its rotatable dihedral angles, preliminary results in vacuo on the conjugated basis used the STO-3G* basis set. At that level, the phi(1), phi(3) potential energy surfaces for phi(2) equal either to 0 degrees or 180 degrees were also explored in the flexible rotor approximation. When phi(2) is equal to 180 degrees, however, for 120 degrees less than or equal to phi(1) less than or equal to 260 degrees the nitrogen proton shuttling to the adjacent carboxyl anion was observed; therefore, a constraint had to be imposed on the N-H bond distance to prevent this artifact. In the phi(2) = 180 degrees potential energy surface computed employing the 6-31G* basis set, in fact, the proton remained on the N atom everywhere. The proton transfer was then studied in vacuo at the HF/6-31G* and 6-31+G* levels and, in contrast to the STO-3G* spontaneous process, the energy barrier turned out to be about 13 kcal/mol. The energy of selected conformers corresponding to minima on the potential energy surface in vacuo was computed with the inclusion of correlation corrections at the MP2 level and in aqueous solution in the polarizable continuum model framework. A sensitive influence of correlation effects on the trans forms with intramolecular H-bonds is observed, while the solvent stabilizes the conformer with the polar groups widely exposed to it. The energy gap among the cis rotamers amounts to similar to 3 kcal/mol, with the energy of the most stable of them in-vacuo similar to 6 kcal/mol higher than that of the trans conformers. The most stable cis structure becomes the second most populated conformer in aqueous solution at the MP2 level, though all the cis conformers are sharply stabilized in solution. A preliminary study in vacuo of the trans-cis interconversion was also carried out.