We determined the regional area of the diaphragmatic zone of apposition (ZAP) as well as the regional craniocaudal extent of the ZAP (ZAPht) of the passive diaphragm in six paralyzed anesthetized beagle dogs (8-12 kg) at residual lung volume (RV), functional residual capacity (FRC), FRC + 0.25 and FRC + 0.5 inspiratory capacity, and total lung capacity (TLC) in prone and supine postures. To identify the caudal boundary of the ZAP, 17 lead markers (1 mm) were sutured to the abdominal side of the costal and crural diaphragms around the diaphragm insertion on the chest wall. Two weeks later, the dogs' caudal thoraces were scanned by the use of the dynamic spatial reconstructor (DSR), a prototype fast volumetric X-ray computer tomographic scanner, developed at the Mayo Clinic. The three-dimensional spatial coordinates of the markers were identified (+/- 1.4 mm), and the cranial boundary of the ZAP was determined from 30-40 1.4-mm-thick sagittal and coronal slices in each DSR image. We interpolated the DSR data to find the position of the cranial and caudal boundaries of the ZAP every 5 degrees around the thorax and computed the distribution of regional variation of area of the ZAP and ZAPht as well as the total area of ZAP. The ZAPht and area of ZAP increased as lung volume decreased and were largest near the lateral extremes of the rib cage. We measured the surface area of the rib cage cephaled to the ZAP (AL) in both postures in another six beagle dogs (12-16 kg) of similar stature, scanned previously in the DSR. We estimated the entire rib cage surface area (Arc = AZAP + AL). The AZAP as a percentage of Arc increased more than threefold as lung volume decreased from TLC to RV, from approximately 9 to 29% of Arc.