The changes in the firing of single motor units (MUs), i.e. the probability for the appearance of an impulse and the instantaneous firing frequency, from m. tibialis anterior and m. triceps surae, were investigated when the respective muscle was agonist or antagonist of the movement, as well as when the movement was performed by the contralateral leg. The changes in MU firing show that the excitability of the alpha-motoneurones is changed 25-45 ms before the front of the ipsilateral voluntary electromyogram. The excitability is increased when the muscle is agonist of the movement, and decreased when it is antagonist. The period of changed MU firing, found by us, corresponds to the duration of the period of increased monosynaptic reflex excitability, preceding the voluntary motor activity (Gerilovsky et al., 1982). Therefore, there are no grounds for assuming that the mechanisms causing these changes differ in the two cases. The results obtained show that the observed changes are specific: they are connected with the organization of the voluntary movement and they do not occur in movement involving the contralateral leg to the MU investigated.