When an excised lung lobe undergoes atelectasis, its shape differs from that when lobar atelectasis occurs in an intact animal: the chest wall deforms the collapsing lobe. In 8 anesthetized dogs in the left lateral position lung volume and transpulmonary pressure was measured during development of atelectasis. Atelectasis of the left lower lobe was then induced with the rest of the lung maintained at FRC [functional residual capacity] and measured lobar volume and translobar (lobar minus esophageal) pressure. Lung and lobar volumes were measured by prebreathing the animal with 88% O2-12% N2, occluding the airway and observing the increase in lung or lobar N2 concentration. When the left lower lobe alone collapsed, translobar pressures were more negative than transpulmonary pressure at the same relative volume when the whole lung collapsed. This pressure difference, which represents the deforming force applied to the lobe minus the pressure costs of deformation, averaged 3 cm H2O at 50% FRC. Infusion of 25 ml of normal saline into the pleural space sharply reduced the difference between translobar pressure during lobar collapse and transpulmonary pressure during lung collapse: this difference was abolished at 80% FRC and halved at 50% FRC. The large effect of the small volume of fluid suggested that deforming forces were largely generated in relatively local areas, such as regions of the chest wall with sharp angulation.