A one-dimensional, self-consistent computer code was used to predict electron temperature and density changes in the lower ionosphere caused by high power, high frequency (HF) radio wave heating. The code was used to obtain first estimates of ionospheric perturbations due to the proposed HF Active Auroral Research Program (HAARP) heating instrument, which is currently in the early phases of construction. Simulations were conducted using both pulses and continuous wave (CW) heating modes, under a variety of conditions. Results indicate that electron temperatures will be increased by up to a factor of 15, and electron densities will be changed by up to 50% relative to natural levels. ¿ American Geophysical Union 1994