Nine hours after the start of treatment with caerulein in rats, an increase in the weight of the pancreas and an increase in serum amylase levels were observed. Likewise, a significant increase in endogenous secretin occurred in rats with acute pancreatitis. A dramatic reduction in the secretion of total protein and amylase was also observed. A partial recovery of this latter effect was achieved after an infusion of high doses of secretin. Under our experimental conditions, the volume of secretion did not vary in caerulein-treated rats wtih respect to controls, either in resting conditions or under secretin stimulation, which indicates that the ductular cells were not significantly affected. Isoelectrofocusing (IEF) and crossed-immunoelectrophoresis (CIE) studies revealed important alterations in the proteins of the pancreatic juice of rats with caerulein-induced acute pancreatitis. Trypsinogen appeared to be particularly affected, showing an increase in the T2 acidic form with an IEP of 4.4 and a decrease in the basic form T3 with an IEP of 8.0, which splits in other forms with a clear antigenic community. A hydrolase was also observed with an IEP of 6.2. In this sense, secretin administration may also be said to induce a significant improvement in established acute pancreatitis, since it tended to normalize the structure and proportion of the proteins secreted.