HF radiation at 3 MHz generated by positive and negative return strokes is presented along with the associated broadband electric fields. The mean duration of the HF associated with negative return strokes was about 140 μs, with minimum and maximum values of 60 and 600 μs, respectively. The durations of the HF associated with the positive return strokes ranged from 5 to 25 ms which is much longer than the duration of HF radiation associated with negative return strokes. It is suggested that at least part of the HF radiation emitted by the return stroke is generated in the collapse of the leader corona sheath during the return stroke. ¿ American Geophysical Union 1994