The study investigated the protective effect of a new synthetic protease inhibitor, E-3123, a 4-guanidinobenzoate methanesulphonate, on the exocrine pancreas in caerulein-induced pancreatitis of rats both in vivo and in vitro. Hyperamylasaemia, pancreatic oedema and congestion of amylase, as well as cathepsin B leakage from lysosomes and malate dehydrogenase leakage from mitochondria, were prevented by infusion of 5 mg/kg.h E-3123 particularly when infused for 2 h before and during 5 μg/kg.h caerulein infusion for 3.5 h. The results indicate that E-3123 plays its protective roles against pancreatitis in the subcellular compartments such as lysosomes and mitochondria, and that such a low molecular weight protease inhibitor as E-3123 may be clinically useful in the treatment of acute pancreatitis.