The perturbation in the signal strength of a VLF transmitter by another transmitter heating the ionosphere on the great circle path to the receiver is calculated for two cases: The NAA-Palmer path heated by the VLF transmitter, NAU, and the Omega Norway-Skibotn path heated by the MF transmitter, ''Heating,'' near Tromsϕ. Comparison implies either that VLF heating is over 100 times more effective than MF heating in terms of the perturbation produced, per RF heating watt, in VLF propagation in the Earth-ionosphere wave-guide or that the observed effect at Palmer is fortuitous or spurious. ¿American Geophysical Union 1991