Motor units (MUs) with low voluntary recruitment thresholds are the first to be activated by transcranial magnetic stimulation. It is not clear, however, if high-threshold MUs can also be activated and if they contribute to motor evoked potentials (MEPs). We therefore studied 11 high-threshold motor units in the first dorsal interosseous muscle of 11 healthy subjects. Voluntary recruitment thresholds ranged from 22 to 41% (29.5 +/- 5.6%; mean +/- S.D.) of maximal muscle force. When MUs were driven at their recruitment thresholds, transcranial magnetic stimuli were applied to the vertex. Peri-stimulus time histograms of MU discharges were constructed. All MUs studied revealed a period of increased firing probability at 19-27 ms after the stimulus (primary peak). Stimulus intensities were lower by 10-57% of the maximal stimulator output than required to produce near maximal MEPs in conventional surface EMG recordings in the same subjects. We conclude that high-threshold MUs can be activated by transcranial magnetic stimulation and that they contribute to conventionally recorded MEPs.