We have used a computer simulation to study ionospheric perturbations produced by gravity waves in the midlatitude F region. The change of amplitude and vertical wavelength of gravity waves with height is taken into account. The presence of plasma diffusion causes the height profile of the wave-induced ionospheric perturbations to be quite different from the forcing gravity waves but does not lead to field alignment of the ionospheric structures. The attenuation of the amplitude of gravity waves with height does not cause the upturning of the phase surfaces of the wave-induced ionospheric perturbations. The upturning of the phase surface of ionospheric perturbations produced by a gravity wave is caused solely by the increase with height of the vertical wavelength of the wave, regardless of whether the gravity wave amplitude increases or decreases with height. The amplitude of gravity-wave-induced ionospheric perturbations is smaller at higher altitudes than at lower altitudes because of the presence of plasma diffusion even if the gravity waves have larger amplitudes at high altitudes. Strong ionospheric density gradients can be produced in the altitude range of 200--400 km. The numerical results are compared with the values predicted by theory. If the gravity waves are uniform in space, the theory gives reasonable estimates of the relative ionospheric density fluctuations. Even when plasma diffusion is present, deviation of the numerical results from the theory is generally <10%. However, if a more realistic gravity wave model in which the amplitude and wavelength vary with height is used in the simulations, significant differences, as much as 50% for daytime ionospheric perturbations, arise between the numerical and theoretical results. ¿ 1998 American Geophysical Union