The possibility that radiative instabilities in stellar atmospheres are driven by transfer of energy processes between stellar gas and the radiation field is considered. Harmonic temperature fluctuations in gray radiative equilibrium atmospheres are introduced, and the linear response of the radiation field to the ensuing Planck-function and opacity fluctuations is investigated. Analytical and numerical calculations are performed, emphasizing the influence of the multidimensional radiative transfer (MRT) effects of opacity fluctuations on the radiative relaxation time as a function of the wavenumber of the perturbations. Quantitative examples are given for stellar atmospheres with solarlike T(eff) and gravitational stratification. It is concluded that, while the MRT effects of B fluctuations generally tend to be stabilizing, the MRT effects of chi fluctuations are optically important for driving radiative instabilities.