The three-dimensional structures of seven 1,2,5-trimethyl-4-amino(amido)piperidines were studied by means of the 1H and 13C NMR spectra. The 3JHH values and the 13C chemical shifts indicate that the substituents in the piperidine ring of all of the 2c,5c,4r isomers have a 2e,4e,5a orientation, while those in the piperidine ring of all of the 2c,5t,4r isomers have a 2e,4e,5e orientation. The conformational change (as a result of ring conversion) 2e,4a,5e → 2a,4e,5a was observed for the 2t,5c,4r isomers on passing from the corresponding amine to the amide; this change is associated with striving of the more bulky amide substituent to become equatorially oriented. Retarded rotation about the C(4)-N bond was observed in the 2c,5t,4r isomer of the 4-(N-phenylbenzamido) derivative; this is explained by steric hindrance due to the 5e-CH3 group. © 1988 Plenum Publishing Corporation.