Summary In the special case of NH 3, the ± K degeneracy of the rotational levels of a symmetrical top is removed, as a consequence of the fact, that the N-atom has two equilibrium positions. The two resulting levels have no dipole moment in zero field, but as non-diagonal elements exist between the two neighbouring states, a linear Stark-effect is found in high fields. In low fields there is a quadratic effect. These circumstances do not affect the determination of the permanent moment from measurements on the electric susceptibility. Scheffers' experiments on molecular beams are discussed and it is shown, that the theory predicts deflections of the beam of the order of magnitude, which is found experimentally.