We studied the endothelial and epithelial cells response in the thin portion of the air-blood barrier to a rise in interstitial pressure caused by an increase in extravascular water (interstitial edema) obtained in anesthetized rabbits receiving saline infusion (0.5 ml/kg x min for 3h). We carried a morphometric analysis of the cells and of their microenvironment (electron microscopy); furthermore, we also studied in lung tissue extracts the biochemical alterations of proteins responsible for signal transduction (PKC, caveolin-1), for cell-cell adhesion (CD31), and of proteins involved in membrane to cytoskeleton linkage (alfa and beta tubulin). In endothelial cells we observed a folding of the plasma membrane with an increase in cell surface area, a doubling of plasmalemma vesicular density and an increase in cell volume. Minor morphological changes were observed in epithelial cells. Edema did not affect the total plasmalemma amount of PKC, beta-tubulin and caveolin-1, while alfa-tubulin and CD-31 increased. In edema, the distribution of these proteins changed between the detergent resistant fraction of the plasma membrane (DRF, lipid microdomains) and the rest of the plasma membrane (high density fractions, HDFs). PKC and tubulin isoforms shifted from DRF to HDFs in edema while caveolin-1 increased in DRF at the expense of a decrease in phosphorylated caveolin-1. The changes in cellular morphology and in plasma membrane composition suggest an early endothelial response to mechanical stimuli arising at interstitial level following a modest (about 5%) increase in extravascular water.