Recent experiments carried out at the Arecibo Observatory in Puerto Rico and the magnetically conjugate location in Argentina suggest a connection between the prevalence and intensity of the interhemispheric propagation of VLF transmissions and HF heating of the ionosphere. During several nights of the study the O-mode CW heater appeared to enhance the coupling of VLF transmissions into the affected region of plasma. These signals presumably then entered plasmaspheric ducts more efficiently and led to conjugate measurements of relatively high amplitude. No such effects were seen with pulsed or X-mode heating. Short time resolution measurements of fading transmissions were marked by transit delays and amplitudes that remained nearly constant as the interhemispheric VLF signals diminished, consistent with the model of a plasmaspheric duct complemented by favorable plasma structures extending well into the F region but producing the majority of the total transit delay at plasmaspheric altitudes. It is also suggested that weak long-delay receptions often observed in these data are most probably prolongitudinal whistlers that are not ducted but which represent a potential contaminant in ducted VLF experiments of all types. ¿ 2001 American Geophysical Union