The therapeutic effect of ulinastatin, an inhibitor of the protease activity, on endotoxin shock was evaluated using 17 Beagle dogs. Single intravenous injection of ulinastatin at a dose of 5,000 or 25,000 U.kg-1 failed to suppress the endotoxin-induced circulatory disturbance but significantly inhibited increases in pulmonary arterial pressure and pulmonary vascular resistance that occur early following administration of endotoxin. Elevation of PGI2, TXA2 and LTB4 by endotoxin shock was significantly inhibited by administration of 25,000 U.kg-1 of ulinastatin. Elevation of the granulocytic elastase activity was inhibited dose-dependently by administration of ulinastatin. The above results indicate that ulinastatin may be a promising drug for the treatment of endotoxin shock.