Variations of the soleus H-reflex were studied during voluntary isometric or anisometric contractions of the tibialis anterior in man. At the onset of isometric contractions 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 potentiation of the H-reflex inhibition is brought about by group I fibres activity, since it was markedly reduced during ischemia of the leg. It persisted during local muscular fatigue, this indicating that Ib fibres from tibialis anterior are not involved and that, by the process of elimination, group Ia fibres must be responsible for the supplementary secondary inhibition. 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 brought about by Ia fibres from tibialis anterior. The secondary potentiation of the inhibition is therefore likely to be produced via the gamma loop.