The temporal evolution of HF-enhanced plasma lines (HFPLs) is investigated theoretically. The HFPLs refer to the radar echoes at frequencies near the sum and difference of the radar frequency and the HF heater wave frequency. These enhanced spectral lines are caused by backscatter of radar signals from plasma waves having a wavenumber of 18 m-1. A nonlinear theory has been developed to explain both the intensity overshoot and altitude expansion of HFPLs observed at Arecibo, Puerto Rico [Djuth and Sulzer, 1989> with good agreement. ¿American Geophysical Union 1990