The longitudinal and transverse components of the dielectric susceptibility tensor are calculated for a system of noninteracting symmetrical top molecules subjected to a strong dc electric field. The calculations are carried out using the extended rotational-diffusion model (J-diffusion). The model predicts a strong dependence of the dielectric spectra on the strength of the dc bias field and on the shape of the molecule. Moreover, the dielectric relaxation behavior at low frequencies (omegatau(gamma)less than or equal to1, where tau(gamma) is the orientation correlation time) is reproduced and the inclusion of inertial effects ensures that optical transparency is regained at very high frequencies (in the far infrared region). (C) 2003 American Institute of Physics.