The present studies were performed to evaluate pancreatic exocrine function in rats during the early stage of acute pancreatitis in two models: one is edematous pancreatitis induced by four subcutaneous injections of 20 μg/kg body weight of cerulein at hourly intervals and the other is hemorrhagic pancreatitis induced by retrograde infusion of 0.4 ml/kg body weight of 3% sodium taurocholate (NaTc) into the pancreatic duct. Secretory studies were performed in vivo under urethane anesthesia at various times after induction of acute pancreatitis. Basal pancreatic fluid secretion was significantly elevated after induction of acute pancreatitis in both the cerulein and the NaTc models, reaching the peak level on postpancreatitic days 1 and 3, respectively. In both models of rats, a stepwise increasing dose of cerulein was unable to cause a further increase in fluid secretion above the basal level, whereas it caused a dose-dependent increase in protein output in both models, al though the responsiveness and the sensitivity were markedly reduced compared with the controls. In contrast to cerulein, secretin caused a dose-dependent increase in fluid secretion in both models of pancreatitis. In cerulein-induced postpancreatitic rats, secretin also caused a dose-dependent increase in protein output and bicarbonate concentration, but it had only a small effect at certain doses in NaTc-induced postpancreatitic rats. These results indicate that basal pancreatic fluid secretion was greatly increased but the secretory response to cerulein stimulation was reduced in acute pancreatitis early after the onset but was not reduced to secretin stimulation and that protein output and bicarbonate concentration were reduced depending on the severity of pancreatitis (NaTc-pancreatitis > cerulein-pancreatitis).