Inflation induces a marked decrease in the lung expanding ability of the diaphragm, but its effect on the parasternal intercostal muscles is uncertain. To assess this effect, the phrenic nerves and the external intercostals were severed in anesthetized, vagotomized dogs, such that the parasternal intercostals were the only muscles active during inspiration, and the endotracheal tube was occluded at different lung volumes. Although the inspiratory EMG activity recorded from the muscles was constant, the change in airway opening pressure ( Pao) decreased with inflation from - 7.2 ± 0.6 cm H₂O at functional residual capacity (FRC) to - 2.2 ± 0.2 cm H₂O at 20 cm H₂O transrespiratory pressure (P <0.001). The inspiratory cranial displacement of the ribs remained virtually unchanged, and the inspiratory caudal displacement of the sternum decreased moderately. However, the inspiratory outward rib displacement decreased markedly and continuously; at 20 cm H₂O, this displacement was only 23 ± 2 % of the value at FRC. Calculations based on this alteration yielded substantial decreases in Pao. It is concluded that in the dog, 1) inflation affects adversely the lung expanding actions of both the parasternal intercostals and the diaphragm; and 2) the adverse effect of inflation on the parasternal intercostals is primarily related to the alteration in the kinematics of the ribs. As a corollary, it is likely that hyperinflation also has a negative impact on the parasternal intercostals in patients with chronic obstructive pulmonary disease.