Results of six-dimensional variational calculations of vibrational energy levels are presented for ammonia using a Hamiltonian expressed in curvilinear internal bond coordinates. A two-dimensional potential energy surface, which was introduced in a preliminary study on the inversion motion, is combined with a surface by Martin et al. Both surfaces are calculated at the aug-cc-pVTZ/CCSD(T) ab initio level. The exact kinetic energy operator is an enlargement of the one used in the previous two-dimensional calculations. Eigenvalues are computed variationally using successive basis set contractions for some symmetric and asymmetric isotopomers of ammonia. (C) 2002 Elsevier Science B.V. All rights reserved.