The properties of the human motor cortex can be studied non-invasively using transcranial magnetic stimulation (TMS). Stimulation at high intensity excites corticospinal cells with fast conducting axons that make direct connections to motoneurones of human upper limb muscles, while low-intensity stimulation can suppress ongoing EMG. To assess whether these cells are used in normal voluntary contractions, we used TMS at very low intensities to suppress the firing of single motor units in biceps brachii (n = 14) and first dorsal interosseous (FDI, n = 6). Their discharge was recorded with intramuscular electrodes and cortical stimulation was delivered at multiple intensities at appropriate times during sustained voluntary firing at approximately 10 Hz. For biceps, high-intensity stimulation produced facilitation at 17.1 +/- 2.1 ms (lasting 2.4 +/- 0.9 ms), while low-intensity stimulation (below motor threshold) produced suppression (without facilitation) at 20.2 +/- 2.1 ms (lasting 7.6 +/- 2.2 ms). For FDI, high-intensity stimulation produced facilitation at 23.3 +/- 1.2 ms (lasting 1.8 +/- 0.4 ms), with suppression produced by low-intensity stimulation at 25.2 +/- 2.6 ms (lasting 7.5 +/- 2.6 ms). The difference between the onsets of facilitation and suppression was short: 3.1 +/- 1.2 ms for biceps and 2.0 +/- 1.5 ms for FDI. This latency difference is much less than that previously reported using surface EMG recordings ( approximately 10 ms). These data suggest that low-intensity cortical stimulation inhibits ongoing activity in fast-conducting corticospinal axons through an oligosynaptic (possibly disynaptic) path, and that this activity is normally contributing to drive the motoneurones during voluntary contractions.