Elastase-induced changes in flow were used to quantify the degradation of lung interstitial elastin. Degassed rabbit lungs were inflated with silicon rubber via airways and vessels. The lungs were cut into 1 cm thick sections. Two chambers were bonded to each section to enclose the interstitium surrounding an arterial segment. Flow of albumin solution (0-5 g/dl) between the chambers was followed by that of the albumin solution with 0.25 g/dl pancreatic elastase solution. Driving pressure was 5 cmH₂0 and mean interstitial pressure (Pm) was either 0 or 10 cmH₂O. Elastase caused an increase in flow in ~70% of the interstitial segments and a reduction in flow in the remaining segments. The elastase-induced response in flow was independent of both albumin concentration and Pm. Leukocyte elastase (5 unit/dl) produced flow responses similar to those of 0.25 g/dl pancreatic elastase. The increased flow of leukocyte elastase was reduced by a subsequent flow with 0.25 g/dl pancreatic elastase but enhanced by a subsequent flow with a 10 fold lower concentration. A change in the order of the elastase flows reversed the concentration-dependent responses. This behavior suggests a complex interaction among the interstitial fibers after degradation by pancreatic and leukocyte elastase. Endogenous elastase-induced increases in interstitial permeability might affect blood-lymph barrier permeability, whereas elastase-induced cessation of flow might be related to the alveolar septal wall destruction observed in emphysema.