The dispersion relation for hydrodynamic waves in an ionosphere with at most a weak magnetic field shows that gravity waves as well as hydrodynamic hybrid waves may be excited in the topside ionosphere of Mars and Venus owing to fluctuations in the solar wind pressure. The gravity wave, which propagates horizontally with a frequency equal to the buoyancy frequency, belongs to the classic branches of acoustic-gravity wave (AGW) mode. The hybrid waves result from coupling between two different hydrodynamic wave modes. One of these modes is the AGW mode, and the other is excited independent of gravity but dependent on the presence of horizontal gradients in the background plasma pressure and density. The latter mode propagates horizontally but can also propagate vertically if there is a vertical gradient in the horizontal velocity. This new mode is therefore called background gradient wave (BGW). The hybrid waves will cause fluctuations with a wavelength of tens of kilometers in the vertical plasma altitude profiles when they propagate vertically. The period of the waves will be of the order around 103 ~ 104 s. Further properties of possible AGW-BGW waves in Mars' and Venus' ionospheres are given. Radio occultation observations at Mars and Venus show electron density fluctuations in the high-altitude ionosphere. The fluctuations are mainly noise, but they may in part be caused by hydrodynamic wave activity. To verify wave activity, more detailed measurements are required and may be obtained with the low-frequency radar planned for the Mars Express mission.