An experimental investigation of the temporal dynamics of the magnetic zenith (MZ) effect associated with ionospheric modification by high-power HF electromagnetic waves is presented. The observed electron temperature enhancement when the heater beam and the UHF radar are directed along the magnetic zenith is at least twice that observed when the heater and radar are both directed vertically. It is shown that the temperature enhancement reaches the stationary state within 10 s after the heater is turned on. Such times (∼5–10 s) are typical for the development of striations with transverse sizes of the order of several meters. Also, the temporal behavior of the ion line spectra is analyzed for the field-aligned and vertical directions of the UHF radar. A new theoretical explanation is suggested for the aspect sensitivity of the electron temperature enhancement that explains the fast manifestation of the MZ effect. Furthermore, it is shown that maximum electron heating is achieved at some intermediate inclination angle of the heater beam between the MZ and the Spitze angle. An estimate of the angle within which the maximum heating effect exists is presented.