We have measured in 8 supine tétraplégic patients the antero-posterior and latero-lateral diameters of the rib cage at two different levels (3rd and 7th rib), the antero-posterior diameter of the abdomen at the level of the umbilicus together with the respiratory airflow. During inspiration the abdominal diameter increases whilst those of the rib cage decrease (in normal subjects all these diameters increase). During expiration the abdominal diameter returns to its resting position whilst the rib cage diameters at first increase above their resting position (with a reduction of the expiratory flow) and then, together with the abdomen, return to their resting position. The expiratory recoil of the chest wall deformed during the previous inspiration is thought to determine the reduction of the expiratory flow observed in these patients. The motion of the latero-lateral diameter at the level of the 7th rib seems to indicate a direct expanding action of the diaphragm on the lower lateral portion of the rib cage. These results allow the analysis of the mechanical action of the diaphragm on the passive rib cage and suggest that the work of breathing in the tétraplégie patients is higher.