In this study, the Authors have provoked experimentally acute pancreatitis in the rat by different methods, in order to find out morphological modifications of the pancreas in the initial stage of the disease. In case of biliary and pancreatic duct obstruction with pure pancreatic reflux, both oedema and inflammatory infiltrations were evident, whereas, in the presence of biliary reflux too, more serious histological features were detected. In conclusion, the first sign of alteration in acute pancreatitis could be represented by the association of intraductal hypertension and pure pancreatic reflux.