The lung-expanding action of the diaphragm is primarily related to the descent of the dome produced by the shortening of the muscle fibers. However, when the phrenic nerves in dogs are selectively stimulated at FRC, the muscle insertions into the lower ribs also move caudally. This rib motion should enhance the descent of the dome and increase the fall in pleural pressure ( Ppl). To quantify the role of this mechanism in determining Ppl during isolated diaphragm contraction and to evaluate the volume-dependence of this role, radiopaque markers were attached to muscle bundles in the midcostal region of the muscle in six animals, and the three-dimensional location of the markers during relaxation at different lung volumes and during phrenic nerve stimulation at the same lung volumes was measured using computed tomography. From these data, accurate measurements of muscle length, dome displacement, and lower rib displacement were obtained. The values of dome displacement were then corrected for lower rib displacement, and the values of Ppl corresponding to the corrected dome displacements were obtained using the measured relationship between Ppl and dome displacement. The measurements showed that phrenic stimulation at all lung volumes causes a caudal displacement of the lower ribs and that this displacement, taken alone, contributes ~ 25% of the Ppl produced by the diaphragm. To the extent that this lower rib displacement is itself caused by Ppl, the lung-expanding action of the diaphragm during isolated contraction may therefore be viewed as a self-facilitating phenomenon.