The development and prevalence of endoscopic retrograde cholangiopancreatography(ERCP) has resulted in great progress in the diagnosis of the disease of the pancreas, and biliary tract. At the same time, however, the rise of serum amylase level has been often point out as the complication of ERCP. To assess the effect of synthetic protease inhibitor, Gabexate mesilate(FOY), in hyperamylasemia following ERCP, the authors carried out a controlled trial in 39 patients who were underwent ERCP under the impression of pancreatic or biliary system disease. Of them 20 patients were allocated to FOY@ treated group and 10 patients to control group. The following results were obtained: 1) There was no significant difference in serum amylase level between FOY treated group and control group before ERCP, however significant difference was observed in seram amylase level at 2 hour and 2 day after ERCP between FOY treated group and control group Before ERCP the serum amylase level was 107. 6 45. 1 I.U./dl in control group and 105. 7+-42.1 I.U./dl in FOY treated group.At 2hour after ERCP the serum amylase level was 186.7 +-150.1 I.U./dl in control group and 103.2+-47.9 I.U./dl in FOY treated group. On 2nd day after ERCP, the serum amylase level was 167.7+-84. 1 I.U,/dl in control group and 109.6+-59.1 I.U./dl in FOY treated group 2) The change in serum amylase levels at 2 hour and 2 day after ERCP by the extent of visualization showed a tendency that a greater extent of visualization was associated with a higher incidence of serum amylase levels. Such a tendency was more pronounced in control group and the mean serum amylase level was significantly higher in the control group. 3) FOY did not cause any serious side effect such as shock, fall of blood pressure, itching, eruption, chest pain etc. in all cases. 4) There was some report that FOY has a relaxant action on the Oddis sphincter and helps the cannulation to the opening of Papilla of Vater, however it more depended upon the skill of the endoscopist, anatomical variation of the Papilla of Vater and FOY did not give any great advantage to the cannulation.