We studied the effect of thrombin on albumin permeability across the endothelial monolayer in vitro. Bovine pulmonary artery endothelial cells were grown on micropore membranes. Morphologic analysis confirmed the presence of a confluent monolayer with interendothelial junctions. Albumin permeability was measured by the clearance of 125l-albumin across the endothelial monolayer. The control 125l-albumin clearance was 0.273 ± 0.02 μl/min. The native enzyme, α-thrombin (10−6 to 10−10 M), added to the luminal side of the endothelium produced concentration-dependent increases in albumin clearance (maximum clearance of 0.586 ± 0.08 μl/min at 10−6 M). Gamma (γ) thrombin (10−6 M and 10−8 M), which lacks the fibrinogen recognition site, also produced a concentration-dependent increase in albumin clearance similar to that observed with α-thrombin. Moreover, the two proteolytically inactive forms of the native enzyme, i-Pr2 P-α-thrombin and D-Phe-Pro-Arg-CH2-α-thrombin, increased the 125l-albumin clearance (0.610 ± 0.09 μl/min and 0.609 ± 0.02 μl/min for iPr2 P-α-thrombin and D-Phe-Pro-Arg-CH2-α-thrombin at 10−6 M, respectively). Since the modified forms of thrombin lack the fibrinogen recognition and active serine protease sites, the results indicate that neither site is required for increased albumin permeability. The increase in albumin clearance with α-thrombin was not secondary to endothelial cell lysis because lactate dehydrogenase concentration in the medium following thrombin was not significantly different from baseline values. There was also no morphological evidence of cell lysis. Moreover, the increase in 125l-albumin clearance induced by α-thrombin was reversible by washing thrombin from the endothelium. The basis for the increased albumin permeability following the addition of α-thrombin appears to be a reversible change in endothelial cell shape with formation of intercellular gaps.