1. The origin of the co-ordination and rhythm of mastication provoked by electrical stimulation of the putamen and corticobulbar pathways was studied in the rabbit.2. Bursts of activity were recorded from the mandibular and hypoglossal nerves and the hypoglossal nucleus, which were in phase with the observed masticatory movements. Discharges occurred alternately in nerves to jaw-opening and jaw-closing muscles.3. The rate of burst discharges was not altered by paralysis, or by large variations in the stimulation frequency.4. Regularly recurring bursts of activity continued to occur in the hypoglossal nucleus in response to random frequency stimulation after severing branchial nerves, cervical nerves and the spinal cord of paralysed rabbits.5. Mechanical deformation of the brain of vascular or respiratory origin was discounted as the origin of the rhythm.6. It is concluded that mastication is controlled by a brain-stem pattern generator which can be activated by adequate inputs from certain higher centres and, as concluded in other studies, from the oral cavity itself.