Comparison of the volume-pressure relation of the respiratory system obtained through different procedures. - The volume-pressure relation of the respiratory system determined from the end-espiratory data during positive pressure breathing is equal to that obtained during the maneuver of voluntary relaxation in 5 out of 6 subjects. The relation determined during negative pressure breathing is significantly to the left of that during relaxation in 3 out of 6 subjects. The relation, during relaxation with closed glottis is equal to that with open glottis: hence the differences among the relations previously described in the literature does not seem to be related to the fact that in some experiments the extrathoracic airways were not subjected to a pressure.