We have studied in cats the discharge pattern in response to lung inflation and deflation of 283 medullary respiratory neurones, 173 being inspiratory and 110 expiratory. The ventral respiratory nucleus, near the nucleus ambiguus, was particularly investigated. The neurones were classified into bulbo-spinal neurones, laryngeal motoneurones and propriobulbar neurones by antidromic invasion from the spinal cord or the vagus nerve (collision test). The bulbo-spinal neurones responded in the same direction as spinal motoneurones in the Hering-Breuer reflexes: depression of inspiratory neurones and facilitation of expiratory neurones by inflation of the lungs. All the expiratory laryngeal motoneurones and some inspiratory laryngeal motoneurons responded in the opposite direction to the Hering-Breuer reflexes: depression of expiratory motoneurones and facilitation of inspiratory motoneurones. The function of propriobular neurones could be inferred from their response to Hering-Breuer reflexes: one group of propriobulbar neurones responded in such a manner as to be linked to the activity of the bulbo-spinal neurones; the other group responded in accordance with the activity of the laryngeal motoneurones.