A detailed conformational analysis was performed on the chair forms of piperidine, N-methylpiperidine, and some methylated derivatives Ž . using Hartree᎐Fock HF and MP2 ab initio methods with several basis sets Ž . from 3᎐21G to 6᎐311qqG** , and the most widely used semiempirical Ž . approaches MNDO, AM1, and PM3 . It was found that the use of polarized basis sets at the HF level is adequate enough for the prediction of conformational preferences in the axialrequatorial equilibrium of the N-R group in piperidines. On the other hand, the inclusion of electron correlation becomes necessary for predicting the axialrequatorial energy differences of the equilibria of the methyl group. Semiempirical methods are not recommended, because AM1 and PM3 predict opposite stabilities to those obtained experimentally and MNDO ring geometries are systematically too flat. The origin of the conformational stabilities was interpreted in terms of the natural bond orbital analysis of the HFr6᎐31G** wave functions. The equatorial preferences in the N-H equilibria is mainly due to lower Lewis energies, although delocalization of the nitrogen lone pair is favored in N-H axial forms. N-Methylation increases the equatorial M-Me preferences, because the Lewis energy of axial N-Me forms increases due to larger 1,3-diaxial interactions. Geometrical trends associated with the delocalization of the nitrogen lone pair and with interactions between the introduced N-R and C-Me groups were discussed and related to the degree of planarity of the six-membered ring by means of the puckering coordinates defined by Pople and Cremer.