Vibration-induced effects on respiration in man were studied by recording the electrical activity (EMG) from the intercostal muscles and the diaphragm. The vibration was applied in an upper thoracic region where inspiratory muscle activity prevailed or in a lower thoracic region where expiratory muscle activity prevailed. The effects were also studied by recording the movements of the thorax and the respiratory air flow. Sustained vibration in the upper region enhanced the activity of the underlying inspiratory muscles and caused an expansion of the rib cage whereas it had little or weak effects on diaphragm-activity or on expiratory intercostal muscles. Sustained vibration in the lower region enhanced the activity of the underlying expiratory muscles, often inhibited the inspiratory activity and caused a depression of the rib cage. It also tended to inhibit the diaphragm activity. It was also found that bilateral vibration, timed by the respiratory movements and alternating between upper and lower regions could aid or counteract the ventilation if it was applied respectively 'in phase' or 'out of phase' with the rhythmical contractions in the underlying muscles. The motor responses described are largely explicable in terms of tonic vibration reflexes (TVR) arising in the inspiratory and expiratory intercostal muscles underlying the vibrators.