Camostate, a synthetic serine protease inhibitor, specifically inhibits the trypsin activity in vitro. Immediately after intravenous administration of camostate (5, 2.5, 0.5, and 0.1 mg/kg body weight/h) the trypsin activity in the biliary-pancreatic juice was diminished dose-dependently in anaesthetized rats. The amylase release was not influenced. Camostate and its metabolites were detected by high-pressure liquid chromatography in the pancreatic juice and tissue; in plasma, only the metabolites were found. Therefore, the inhibitory action of camostate on the trypsin activity in the biliary-pancreatic juice may be due to penetration of camostate into the juice.