In conscious subjects, the occlusion pressure measured in the initial part of the inspiration (up to 200 ms) appears to be a valid index of output of the respiratory centres. To interpret any relative change in occlusion pressure, the shape of the occlusion-pressure wave must remain absolutely constant. The purpose of this study was to test the hypothesis that the shape of occlusion-pressure wave does not change during exercise. However, we found that the shape changed significantly during cycle incremental-load exercise in five healthy subjects and in 12 of 17 patients with chronic obstructive pulmonary disease. The major change appeared during the second half of the exercise and mainly during the last workloads. This study shows that it is necessary to take into consideration this form change to interpret any relative change in mechanical transforms of the inspiratory neural output such as pressure at 0.1 s (P0.1) and mean inspiratory flow (VT/TI) or their ratio. This finding confirms, in direct fashion, the expected form change from previous data in which P0.1 is thought to measure the rate of change in chest wall muscle pressure (Pmus) over a segment somewhat removed from the onset of neural inspiration.