To assess the coupling between a particular hemidiaphragm and the individual lungs, the left and right phrenic nerves were separately stimulated in anesthetized dogs and the mean changes in pleural pressure over the two lungs were evaluated by measuring the changes in airway opening pressure (Pao) in the two bronchial trees. Stimulation induced a fall in Pao in both lungs. However, Pao in the contralateral lung was only 65% of that in the ipsilateral lung. Thus, although the canine ventral mediastinum is a delicate structure, it sustained a significant pressure gradient. The hypothesis was then considered that this gradient was allowed to develop through the stretching and stiffening of the mediastinum caused by the descent of the diaphragm, and it was tested by measuring Pao in the two lungs during isolated, unilateral contraction of the inspiratory intercostal muscles. In this condition, Pao in the contralateral lung was 92% of that in the ipsilateral lung. A model analysis of the respiratory system led to the estimate that mediastinal elastance was revsim 25 times greater during hemidiaphragmatic contraction than during unilateral intercostal contraction. These observations indicate that (1) a particular hemidiaphragm has an expanding action on both lungs; and (2) during contraction, however, it makes the mediastinum stiffer so that the pressure transmission from the ipsilateral to the contralateral pleural cavity is reduced. These observations imply that the mediastinum may play a significant role in determining the pressure-generating ability of the diaphragm.