To study the relation between expression of the pancreatitis-associated protein (PAP) and severity of pancreatitis in rats, different degrees of experimental pancreatitis were induced by a 1-, 3-, or 5-h cerulein infusion (5 micrograms kg-1 h-1). This treatment decreased pancreatic volume secretion to below 10%. Immediately after infusion, the secretion rate increased to approximately 50% of control. Within 1 day, volume and bicarbonate secretion rates were not different from controls. At this point, protein secretion amounted to 30% of control, but only in animals receiving the 3- or 5-h dose. The values increased to 40-60% within 3 days. In all groups, the isoenzyme pattern was not influenced by the cerulein treatment. One day after induction of pancreatitis, the PAP was found in pancreatic juice in concentrations related to the dose of cerulein given. By immunohistochemical techniques, the protein was localized over acinar cells, but was not detectable in interstitial tissue, islets, or in the healthy exocrine pancreas. Pathomorphologic alterations in the pancreas were quantified by a scoring system. One and 2 days after the treatment, a more severe pancreatitis and more elevated levels of PAP were found in animals treated with the higher dose of cerulein. It is concluded that PAP is expressed in the pancreas in relation to the severity of cerulein-induced pancreatitis.