Using the HF, MP2, and DFT methodologies in the GAUSSIAN 94 set of quantum chemistry codes with a 6-311G** basis set, we examine theoretically the internal rotation and inversion of the amino group in ethylamine. An analytical potential energy surface corresponding to energy as a function of phi, the dihedral angle of the lone-pair of electrons on the amino group, and tau, the angle between the amine plane and the C-N axis, for these motions is generated by fitting the energy determined at 109 geometries to sums of products of Bessel functions of the first kind, J(m), and cos(m phi). The surface depicts several special points : the minimum energy trans conformer, the two gauche conformers, the three first-order transition states for internal rotation of the amino group about the C-N bond, the two first-order transition states for the inversion of the amino group, and the three second-order transition states also for the inversion motion. The results of a related study on the internal rotation of the ammonium group about the C-N bond in the ethylammonium ion are also interspersed in this report. Both nondeuterated and several deuterated isotopomers of both ethylamine and ethylammonium ion are considered.