Functional capacities of the lungs and thorax in beagles taken to high altitude as adults for 33 mo or in beagles raised from puppies at high altitude were compared with functional capacities in corresponding sets of beagles kept simultaneously at sea level. Comparisons were made after reacclimatization to sea level. Lung volumes, airway pressures, esophageal pressures, CO diffusing capacities (DLCO), pulmonary blood flow, and lung tissue volume (Vt) were measured by a rebreathing technique at inspired volumes ranging from 15 to 90 ml/kg. In beagles raised from puppies we measured anatomical distribution of intrathoracic air and tissue using X-ray computed tomography at transpulmonary pressures of 20 cm H2O. Lung and thoracic distensibility, DLCO, and Vt were not different between beagles that had been kept at high altitude for 33 mo as adults and control subjects kept simultaneously at sea level. Lung distensibility, DLCO, and Vt were significantly greater in beagles raised at high altitude than control subjects raised simultaneously at sea level. Thoracic distensibility was not increased in beagles raised at high altitude; the larger lung volume was accommodated by a lower diaphragm, not a larger rib cage.