To examine the effect of abdominal distension upon the actions of both rib cage and abdomen, we made serial determinations of tidal volume with a chest wall volume-displacement method in 8 pregnant women. Enhancement of tidal volume, long recognized in pregnancy, was achieved usually by augmentation of rib cage volume displacement. By contrast, abdominal volume displacement during quiet breathing is not altered in a predictable fashion by the gravid state. Given these findings, we hypothesize that the increased diaphragmatic contraction of pregnancy is accompanied by the transmission of that force to the lower rib cage via the area of apposition and that diaphragmatic contraction accounts for enhancement of the tidal breath. Diminished abdominal compliance might contribute to the augmentation of rib cage volume displacement as well. Konno-Mead diagrams suggest that this hypothesis is true in some, but not all, subjects.