An internally consistent theoretical framework is developed to study the field line oscillations in the realistic magnetospheric magnetic field using the Mead and Fairfield (1975) model. The nondipolar contributions are numerically computed for the fundamental period of the modes that would reduce to the localized toroidal and poloidal modes described by Cummings et al. (1969) in the dipole limit. It is shown that the nondipolar contributions are not significant at the geostationary orbit but become large further out in the magnetosphere. The nondipolar contributions are very different for the two modes. The situation becomes very much more complicated in the dawn/dusk region where a continuous range of periods exist depending on the orientation of the field line oscillation. ¿ American Geophysical Union 1989