Fluid and kinetic theory calculations predict that the microwave beam from a solar power satellite could be intense enough to cause large, but localized changes in the properties of the lower ionosphere by ohmic heating. A 5-GW system, radiating a peak flux of 23 mW/cm2 at a frequency of 2.45 GHz, could raise the electron temperature by a factor of 2–3, increase the E region density by 10–20%, and cause up to a 50% reduction in the D layer electron concentration. Doubling the flux could lead to a fivefold temperature rise and a 40% increase in the E region density. Since power is absorbed from the beam and heats the ionosphere at a rate that is proportional to the ratio of the flux to the square of an effective frequency, the effects of microwave heating can be studied at much lower power fluxes with high-frequency radio waves. To investigate these effects, a series of ionospheric heating experiments were conducted at Platteville, Colorado, using the high-frequency, high-power facility. A pulsed ionosonde probe, located below the most intense portion of the high-power beam, was used to observe the changes in the D and lower E regions. Phase and amplitude measurements were recorded during both CW and pulsed heating. Prompt (less than 50 ms) and sustained changes were observed in both the phase and attenuation. Perhaps most interesting are the large phase variations of several cycles at 3.4 MHz that were seen following turn-on and turn-off of a 5.2-MHz heating wave. These changes occur relatively slowly, but steadily, in about 60–90 s. The observed changes in the probe amplitude and phase have been compared with theoretical estimates derived by integrating the coupled equations for power flux, electron temperature, and density as a function of altitude and time for both time-varying and steady heating fluxes. The agreement is generally close, considering the differences between the ambient electron density model and the actual distributions.