Recent developments in the treatment of acute pancreatitis have focused on the importance of early determination of the severity of an attack. Measuring levels of activation peptides from pancreatic proenzymes seems to be one way to predict severity. Levels of the activation peptide from procarboxypeptidase B, in both serum and urine on admission, have been shown to correlate to the outcome. To be able to interpret levels of this peptide in serum and urine under normal and in various acute abdominal conditions, we need knowledge about its turnover in the circulation. Procarboxypeptidase B, active carboxypeptidase and the activation peptide were therefore purified from porcine pancreatic juice. These proteins were labelled with 125I or 131I and their turnovers were studied in vivo in the pig. The proenzyme and the activation peptide were eliminated without interaction with any substance in the circulation. The active enzyme was to some degree bound to a substance with a molecular mass of 10-20 kDa. Active CPB was eliminated more slowly than proCPB and the activation peptide. Five percent of the activation peptide was detected nondegraded in the urine. After intraduodenal administration of the activation peptide there was no sign of the peptide in the urine.