Increased activity of various proteases is observed in both human and experimental pancreatitis; however, the information on the effects of specific protease inhibitors on the disease is limited. In this study we show that a novel elastase inhibitor, guamerin-derived synthetic peptide (GDSP), improves the parameters of cerulein-induced acute pancreatitis in the rat. The effects of GDSP on pancreatic weight, serum amylase and lipase, morphologic changes in the pancreas, neutrophil infiltration, and nuclear factor kappa B (NF-kappa B) activation were measured in rats infused with supramaximal dose of cerulein (5 (g/kg/h) for 6 h. The effects of GDSP were also measured on superoxide formation by activated human neutrophils. The effects of GDSP were compared with those of another elastase inhibitor, elastatinal. GDSP significantly inhibited edema formation, neutrophil infiltration, acinar cell damage, and plasma lipase and amylase increases caused by cerulein. GDSP also completely inhibited superoxide formation in the human neutrophils stimulated by N-formyl-methionine-leucine-phenylalanine (fMLP) or 12-O-tetradecanoylphorbol-13-acetate (TPA). Elastatinal had some of the same effects as GDSP but was less potent and effective. These results demonstrate a beneficial effect of GDSP, a novel specific elastase inhibitor, on the development of rat cerulein pancreatitis.