The effect of a novel synthetic trypsin inhibitor, 4-sulfamoylphenyl 4-guanidinobenzoate methanesulfonate (ONO-3307), on severe acute pancreatitis was studied by changing its timing, frequency, and dose in trypsin-taurocholate-induced acute experimental pancreatitis in rats. Rats were divided into four groups according to difference of ONO-3307 administration: group A, 2 mg/0.5 ml of ONO-3307 s.c. 1 h before and after induction of pancreatitis; group B, 2 mg/0.5 ml s.c. 1 and 3 h after; group C, 4 mg/1 ml s.c. 1 h before; group D, 4 mg/1 ml s.c. 1 h after. The survival rate at 24 h was significantly improved in group A (75% in A vs. 17% in control; p < 0.01) and in group B (57 vs. 29%; p < 0.05), but not in group C or D. Amylase and immunoreactive trypsin in serum and ascites of the treated were significantly lower than those of controls in both groups A and B. The survival rates were improved dose dependently when ONO-3307 was administered 1 h before and after induction of pancreatitis. ONO-3307 showed favorable effects on the initial stage of severe acute pancreatitis when given in divided doses to maintain the effective serum levels.