Recent results on the comparative efficiencies of VLF versus HF heating of a typical nocturnal midlatitude ionosphere have been extended. The effects of varying the magnetic latitude and elecron density profile of the ionosphere are investigated. Our results indicate that the superior heating efficiency of VLF vs HF radiation and the resulting perturbation on VLF propagation is a latitude-dependent phenomenon, the effects being accentuated at low magnetic latitude and reversed at polar latitudes. The marked superiority of VLF heating is also found to be a feature of nocturnal ionospheres. VLF and HF radiation produce similar heating effects in ionospheres typical of summer daytime conditions. The optimum design of experiments for the study of the perturbation of VLF propagation by VLF heating transmitters is discussed. ¿ American Geophysical Union 1992