The effects on lung mechanics of equal (37%) reduction in total lung capacity (TLC) by rib cage or abdominal restriction were studied in 10 healthy males. Lung recoil pressure (Pst) was simultaneously measured from three sites in the esophagus. This also provided an estimate of the vertical pleural pressure gradient (PPG). Deformation of the right hemithorax was quantified by roentgenograms in three subjects. At the same lung volume, abdominal restriction decreased lung height and increased anteroposterior diameter compared with the control case, whereas rib cage restriction had opposite effects. Maximum expiratory flow increased equally with both types of restriction, and average Pst increased equally with both types of restriction. There was a significant correlation between degree of TLC reduction and increase in Pst that was similar for both types of restriction. This study indicates that changes in lung mechanics depend primarily on the amount of volume reduction and not on the type of deformation producing the volume decrease.