The frequencies of the three large amplitude modes of dimethylamine (DMA) were analyzed using a three-dimensional (3D) model, the independent variables of which are the two torsional angles and the CNC symmetric bending angle. For this purpose, the potential energy surface and the kinetic parameters of the vibrational Hamiltonian were determined using fully optimized ab initio calculations performed at the MP2/6-311G(d,p) and MP4/6-311G(d,p) levels on 150 nuclear conformations. The positions of the two first hot bands were also calculated. The three fundamentals, 255.4, 216.9, and 409.8 cm(-1), as determined with the MP4/6-311G(d,p) approximation, are in a good agreement with the experimental frequencies of 256.3, 219.4, and 383 cm(-1), respectively. Torsional frequencies were compared with those obtained from a previously published two-dimensional model. The calculations confirm that the 3D model is indispensable for obtaining accurate band separations for the two different torsional modes.