The presence of blood proteins and excess liquid in the airway lumen during airway inflammation may be secondary to extravasation and elevation of subepithelial hydrostatic pressure. This study examines how hydrostatic pressures of 5-20 cm H2O affect hydraulic conductivity and macromolecular permeability of primary cultures of bovine tracheal epithelium. Hydraulic conductivity was not altered by transepithelial pressure gradients of up to 20 cm H2O directed from the mucosal to serosal side of the tissue (m-s). By contrast, a 20-cm H2O s-m pressure resulted in a marked increase in hydraulic conductivity with the critical pressure lying between 10 and 20 cm H2O. Electrical conductance (i.e., permeability to ions) was not altered by m-s pressure gradients, or by a 5-cm H2O s-m gradient, but was increased by s-m pressures > or = 10 cm. Fluxes (s-m and m-s) of fluorescein and fluorescent dextrans (70 and 2000 kDa) were not altered by pressures of up to 20 cm H2O m-s. By contrast s-m pressure gradients of 20 cm H2O dramatically increased the s-m fluxes of these probes. The increases in flux were completely reversible. The results indicate that s-m pressure gradients greatly increase the hydraulic conductivity of airway epithelium by creating pores with an effective diameter greater than 54 nm.