The inspiratory intercostal muscles elevate the ribs and thereby elicit a fall in pleural pressure (Ppl) when they contract. In the present study, we initially tested the hypothesis that this Ppl does, in turn, oppose the rib elevation. The cranial rib displacement (Xr) produced by selective activation of the parasternal intercostal muscle in the fourth interspace was measured in dogs, first with the rib cage intact and then after Ppl was eliminated by bilateral pneumothorax. For a given parasternal contraction, Xr was greater after pneumothorax; the increase in Xr per unit decrease in Ppl was 0.98 ± 0.11 mm/cm H₂O. Because this relation was similar to that obtained during isolated diaphragmatic contraction, we subsequently tested the hypothesis that the increase in Xr observed during breathing after diaphragmatic paralysis was, in part, the result of the decrease in Ppl, and the contribution of the difference in Ppl to the difference in Xr was determined by using the relation between Xr and Ppl during passive inflation. With diaphragmatic paralysis, Xr during inspiration increased ~ threefold, and 47 ± 8 % of this increase was accounted for by the decrease in Ppl. These observations indicate that: 1) Ppl is a primary determinant of rib motion during intercostal muscle contraction; and 2) the decrease in Ppl and the increase in intercostal muscle activation contribute equally to the increase in inspiratory cranial displacement of the ribs after diaphragm paralysis.