The specific role of solid extracellular matrix components in opposing development of pulmonary interstitial edema was studied in adult anesthetized rabbits by challenging the lung parenchyma with an intravenous injection of a bolus of collagenase or heparanase. In 10 rabbits, pulmonary interstitial pressure (P_ip) was measured by micropuncture in control and up to three hours after collagenase or heparanase i.v. injection. With respect to control (P_ip= -9.3 ± 1.5 cmH₂O, n=10), both treatments caused a significant increase of P_ip and of the wet weight to dry weight lung ratio. However, while tissue matrix stiffness was maintained after 60 min of collagenase, as indicated by the attainment of a positive P_ip peak (P_ip= 4.5 ± 0.3 cmH₂O, n = 5), this mechanical response was lost with heparanase (P_ip= -0.6 ± 1.3 cmH₂O, n = 5). Biochemical analysis performed on a separate rabbit group (n = 15), showed an increased extraction of uronic acid with both enzymes, indicating a progressive matrix fragmentation. Gel chromatography analysis of the proteoglycan (PG) families showed that 60 min of both enzymatic treatments left the large molecular weight PGs (versican) essentially unaffected. However, the heparan-sulphate PG fraction was significantly cleaved, as indicated by a significant increase of the smaller PG fragments with heparanase, but not with collagenase. Hence, present data suggest that the integrity of the heparan-sulphate PGs is required to maintain the three-dimensional architecture of the pulmonary tissue matrix and in turn, to counteract tissue fluid accumulation in situations of increased fluid filtration.