Vibration of 100 Hz was delivered longitudinally onto the sternum of anaesthetized cats and rabbits. Vibration consistently reduced the tidal volume by 10-15% without altering the end-expiratory point, and occasionally reduced the respiratory rate. Vibration applied during inspiration reduced the tidal volume as much as if delivered over several breaths. Expiratory vibration did not alter the course of the expiration, nor the volume of the following inspiration. The inhibition of inspiration was unaffected by deafferentation of chest wall skin, bilateral vagotomy, bilateral division of the phrenic nerves and low thoracic spinal transection. Spinal transection above the thoracic cord (C8/T1) abolished usual responses to vibration. The receptors for this reflex probably lie in the chest wall. Vibration inhibited the development of alae nasi tension during inspiration indicating that supraspinal reflex loops were involved. A role for intercostal muscle spindles is suggested.