Variations of the soleus H reflex were studied during voluntary contractions of the tibialis anterior in man. At the onset of contraction there was a weak inhibition of the soleus H-reflex which was not related to the force of the tibialis anterior contraction. 110 msec after the onset of the EMG activity the inhibition became secondarily more marked and was then related to the force of the contraction. This secondary potentation of the H-reflex inhibition is brought about by group I fibres activity, since it was markedly reduced during ischemia of the leg. It is concluded that the early inhibition is only due to suprasegmental activity whereas during the secondary part of the inhibition, there is a supplementary inhibitory action likely brought about by Ia fibres tibialis anterior. The secondary potentiation of the inhibition is therefore likely to be produced via the gamma loop.