In 85 patients undergoing aorto-coronary bypass for atherosclerotic coronary disease, we measured the antithrombin III activity levels and the thrombin-antithrombin III complex concentrations in blood from the pulmonary and the radial arteries, taken before the aorto-coronary bypass procedure, with the aim of investigating the role of the pulmonary endothelium in the metabolism of the inhibitor. Results showed significantly lower mean antithrombin III activity levels, expressed as a percentage of normal plasma, in blood from the radial artery with respect to levels from the pulmonary artery (0.78 +/- 0.12 versus 0.80 +/- 0.12, P<0.0001), while no significant difference was found in thrombin-antithrombin III complex concentrations. The results seem to show that the pulmonary endothelium contributes to the antithrombin III metabolism with a 0.023 breakdown rate, corresponding to about a 0.1 fraction of the reported 0.22-0.25 total body catabolic rate, as well as the pulmonary endothelial surface (50-70 m2) corresponding to about a 0.1 fraction of the peripheral vessels' endothelial surface (500-700 m2). The data support the hypothesis of a main endothelial catabolism of antithrombin III.