THE ACTIVATION of large quantities of trypsin is considered to be of major importance in the pathogenesis of acute pancreatitis.1 In animals,2-5 as well as in patients,4,6-11 the course of inflammatory disorders of the pancreas has been favorably influenced by an antitryptic agent (Trasylol). It appears that this drug inhibits the formation of trypsin and other proteolytic enzymes.6 Since in previous investigations use of this compound was followed by a decrease of the mortality in experimentally produced pancreatitis in animals, it could be anticipated to have a prophylactic action as well.4,12 This study was designed to evaluate the prophylactic effect of its antitryptic activity in dogs which were submitted to experimental pancreatitis. Materials and Methods Adult mongrel fasting dogs weighing approximately 15 kg (33 lb ) were used. In a control series, acute hemorrhagic pancreatitis was produced in 20 dogs by a modified Elliott technique.13