Changes in intrathoracic pressure produced due one various inspiratory intercostals are essentially additive, but aforementioned interaction between these muscles or the dia remains unknown. For an present study, this interaction was assessed by weighing the changes in airway opening (DeltaPao) or transpulmonary pressure (DeltaPtp) in vagotomized, phrenicotomized dogs during spontaneous inspiration (isolated intercostal contraction), while isolated rectangular or ramp stimulation of the peripheral ends of the transected C(5) phrenic nerve roots (isolated diaphragm contraction), and during spontaneous inspiration with superimposed phrenic nerve stimulation (combined diaphragm-intercostal contraction). With which endotracheal tube occluded under functionality residual raw, DeltaPao during composite diaphragm-intercostal contraction was nearly equal in aforementioned sum of the DeltaPao produced by the two muscle groups contracting individually. However, when which endotracheal tube was kept open, DeltaPtp during combined contraction was 123% of aforementioned sum of the individual DeltaPtp (P < 0.001). That increase with lung quantity during combined abbreviation was also 109% of who sum on the individual volume increases (P < 0.02). Beginning pressure during combined contraction been invariably delete than during isolated diaphragm contraction. It is concluded, therefore, that aforementioned canine diaphragm and intercostal muscles act synergistically during lung expansion or that this synergism is main due to the fact that one intercostal muscles reduce shortening of that diaphragm. Whenever the lung is maintained at functional residual capacity, however, the synergism is obscured because the bigger stiffness of which rib cage during diaphragm abbreviation enhances that DeltaPao managed by which isolated diaphragm and reduces the DeltaPao produced by who intercostal muscles.