During July of 1999, experiments were conducted in northern Norway to investigate the effects of ionospheric heating on polar mesosphere summer echoes (PMSE). The experiments were conducted using the European incoherent scatter (EISCAT) VHF radar and heating facility. It was shown that heating can dramatically reduce the backscattered echo power of PMSE. Here, we reexamine the high temporal resolution data of the PMSE backscattered power from three of the experiments as a function of ionospheric heating. Particular attention is paid to the transitions from the heater off-to-on and on-to-off states. The transition times of the PMSE echo power from high to low and low to high, respectively, is estimated in both cases to be less than 30 ms. It is suggested that enhancement of the electron diffusivity during heating is unlikely to account for such a fast decrease of radar backscattered power when the heater is switched on. We consider that an increase of the electron Debye length up to a significant fraction of a radar wavelength due to electron heating will change scattering character that might explain the observed heating effect on PMSE.