This research is motivated by observations of oscillations in the thermosphere and ionosphere at periods close to those associated with planetary waves (PW) or normal modes of the lower atmosphere. Herein a 2-D linear steadystate numerical model is used in conjunction with a hybrid gravity wave (GW) parameterization to examine the role of GW interactions in the penetration of westward propagating planetary waves from the mesosphere into the lower thermosphere. Results are presented for the quasi 2-day and quasi 16-day waves previously studied using the same model without explicit parameterization of GW influences. It is shown that the horizontal winds associated with the PWs result in modulation of upward propagating internal gravity waves, providing an insitu source of periodic forcing in the mesopause region. This forcing is shown to produce separate amplitude peaks in the PW horizontal wind fields in the lower thermosphere apart from the primary response in the mesosphere. Horizontal winds associated with the GW-modified 2-day wave extend well into the thermosphere up to 140 km, primarily at lower latitudes, whereas the GW-modified 16-day wave is enhanced in the 80--120 km altitude range. The phase structure of the modified waves are consistent with the concept of gravity wave filtering by a longitudinally varying planetary wave, with an anticorrelation between the phase of the mesospheric and thermospheric amplitude peaks. The roles of wave stress and eddy diffusion are analyzed and variations of the strength of the meridional GW source are considered. ¿ 1999 American Geophysical Union