1. The hypothesis that a transcortical loop can be activated by electrical stimulation of low-threshold muscle afferents was tested. The effect of these afferents on the excitability of motoneurones was measured with the monosynaptic spinal reflex (H-reflex).2. Four monkeys were trained to maintain a constant tonic activity in the soleus muscle so that the amplitude of evoked H-reflexes was constant. The intensity of conditioning stimuli was just subthreshold for direct or reflex electromyographic responses. The intensity of the test stimuli was adjusted to evoke an H-reflex of maximal amplitude. The amplitude of the H-reflex was recorded for different intervals between conditioning and test stimuli (10-1000 msec).3. The excitability curve obtained showed three components: (1) an early excitatory process, F1, at intervals of 10-20 msec, (2) a late excitatory process, F2, at intervals of 40-80 msec and (3) a short latency depression of about 400 msec duration, on which F1 and F2 were superimposed.4. F2 was selectively abolished during cooling of the contralateral motor cortex, after an irreversible lesion of the motor cortex, and after pyramidotomy; however, F1 and the inhibition remained unchanged.5. The conduction time from the tibial nerve to the somatosensory cortex (SI), the cortical delay between SI and motor cortex, and the conduction time from motor cortex to the soleus muscle were measured in an anaesthetized animal. The sum of these values as an estimate of the transcortical loop time was 5 msec shorter than the latency of F2.6. It is concluded that a transcortical loop can be activated by electrical stimulation of low-threshold muscle afferents and, by analogy, also by mechanical perturbations applied during a motor task.