1. The effect of magnetic stimulation of the human motor cortex on the excitability of soleus, tibialis anterior and flexor carpi radialis motoneurones was investigated by H reflex testing in ten healthy subjects.

2. At rest, an early facilitation of the flexor carpi radialis and tibialis anterior H reflexes was always seen, whereas a similar early facilitation of the soleus H reflex was seen in only two out of seven subjects. For all three motoneuronal pools the facilitation was curtailed 1-5 ms later by an inhibition which lasted for another 3-4 ms. In five subjects an inhibition without any evidence of an earlier facilitation was seen for the soleus H reflex.

3. The intensity of the magnetic stimulation was subsequently decreased so that it had no effect on the H reflex at rest. When the subject then performed a voluntary agonist contraction a facilitatory effect with an early onset and a duration of 20-25 ms was observed for all three muscles. When the subject performed a voluntary antagonist contraction an inhibition was seen for the soleus H reflex with an onset 1-3 ms later than the facilitation. This is interpreted as resulting from the excitation by the magnetic stimulus of corticospinal neurones voluntarily activated in relation to the given motor task.

4. The initial part of the facilitation was significantly smaller during co-contraction of both agonists and antagonists than during isolated agonist contraction.

5. Whereas the early part of the facilitation always occurred during plantarflexion when the H reflex was conditioned by magnetic stimulation, this was never the case when it was conditioned by electrical stimulation of the cortex with the stimulus regimes used in these experiments.

6. It is suggested that the early part of the facilitation observed during agonist contraction is caused by activation of corticomotoneuronal cells projecting to the agonist motoneuronal pool and that the inhibition observed during antagonist contraction is caused by activation of corticospinal cells projecting both to the antagonist motoneuronal pool and la inhibitory interneurones to the agonist motoneuronal pool. The smaller size of the earliest part of the facilitation observed during co-contraction in relation to agonist contraction suggests a different cortical control of the two tasks.