The effect of the decapeptide cerulein on exocrine pancreatic function was compared with pancreozymin by means of intraindividual double stimulation. 100 ng/kg bw/45 min of synthetic cerulein led to a greater enzyme excretion than 2 U/kg bw pancreozymin. This increase in enzyme secretion was independent of whether the pancreas function was normal or disordered. In the selected experimental procedure, cerulein increased the excretion of enzymes (chymotrypsin, trypsin, lipase, amylase) whether it was infused in connection with or before an injection of pancreozymin. Cerulein can therefore completely replace pancreozymin in the diagnosis of pancreatic function.