Changes of respiration caused by chest wall vibration were studied in 13 patients with cervical cord lesions. Vibrators applied on the chest wall were intermittently triggered by the patient's respiratory movements. The vibrators were triggered to be applied in three ways. 1) Upper vibrations were bilaterally applied during inspiration on the 2nd or 3rd parasternal intercostal spaces (insp "in-phase" vibration). 2) Lower vibrations were bilaterally applied during expiration on the 7th-to 10th intercostal spaces anterior to midaxillary lines (exp "in-phase" vibration). 3) Inspiratory and expiratory vibrations were combined to produce alternating in phase vibration. Both inspiratory and expiratory vibrations increased tidal volume (VT), but the alternating in-phase vibration increased it to significantly higher values (P less than 0.05). Minute volume of ventilation (VE l/min) also increased with insp or exp in-phase vibration. The alternating in-phase vibration increased VE l/min to even higher values (P less than 0.05). The responses described probably depend on segmental vibration reflexes arising in the inspiratory and expiratory intercostal muscles underlying the vibrators. The alternating in-phase vibration may well be useful to improve the breathing in patients with respiratory insufficiency.