The effect of procaine microinjection into the ventromedial portion of the medullary reticular formation on forelimb muscle activity evoked by electrical mesencephalic locomotor region and natural vestibular stimulation has been investigated in the decerebrated guinea-pig. This injection is followed by a reversible increase of the threshold of mesencephalic locomotor region stimulation necessary for the activation of muscle rhythmic activity. Procaine injection is accompanied by reduction of vestibular influence on flexor muscle activity evoked by electrical cutaneous and mesencephalic locomotor region stimulation. Vestibular influence on extensor muscle activity remains unchanged after the injection. The results indicate that medial medullary reticular formation is the site of the convergence of mesencephalic locomotor region and vestibular activity. It is suggested that the vestibular system contributes to the modulation of reticulospinal activity coupled with the initiation and control of locomotion.