Measurements were made of the effect of pulmonary vascular distention on static volume-pressure relations of excised cats' lungs filled either with saline to minimize surface forces, or inflated with gas. Only slight changes of questionable significance occurred in elastic behavior of the lungs during deflation when pulmonary vascular pressure was increased from 0 to 16 cm H2O. Measurements were also made of quasi-static pulmonary vascular volume-pressure relations at different lung volumes. The overall slope of the vascular volume-pressure curve in saline-filled lungs was greatest at a moderate lung volume and was slightly less when the lungs were either collapsed or highly distended. The vascular volume-pressure curve was considerably flattened in gas-inflated lungs at airway pressures above 10 cm H2O. During filling and emptying of the vascular system there were slight but definite changes in equilibrium volume of the lung. Hysteresis of the vascular system was an invariable finding. Regardless of the complex mechanism underlying hysteresis, it can be explained if all units of the system are assumed to have sigmoid volume-pressure curves. The findings of these experiments illustrate the highly complex interrelations of the pulmonary blood vessels and lungs, particularly when surface forces are taken into account. Note: (With the Technical Assistance of Martha McLaughlin) Submitted on May 29, 1958