The effect on endogenous beta-endorphins of a new synthetic protease inhibitor was studied in acute pancreatitis. Pancreatitis was induced by the injection of autologous bile mixed with trypsin into the main pancreatic duct after ligation of the accessory duct. Plasma beta-endorphin concentrations and cardiovascular function were measured. Ten dogs (control group) were given 10 ml/kg/h of lactate Ringer's solution intravenously beginning 1 h before the induction of pancreatitis and continuing throughout the experiments. Six dogs received an intravenous infusion of 3 mg/kg/h of a new synthetic protease inhibitor, E-3123 (4-(2-succinimidoethylthio)4-geranidinobenzoate methanesuLfonate), in lactate Ringer's solution soon after the induction of pancreatitis. Plasma beta-endorphin concentrations in the control group increased significantly. However, plasma beta-endorphin levels in the protease inhibitor group did not increase as in the control group. The protease inhibitor infusion improved hypotension, myocardial depression, and plasma lactate, suggesting that the inhibitory effect of the protease inhibitor on beta-endorphin release contributed to the improvement.