1. Caerulein displayed a potent stimulant action on pancreatic secretion in the dog. Threshold doses were 1-5 ng/kg by rapid intravenous injection, 0.25-1 ng/kg per min by intravenous infusion and 50-100 ng/kg by subcutaneous injection. There was a conspicuous increase not only in the volume flow of pancreatic juice but also in the output of solid constituents of the juice and of amylase. However, continuous stimulation of pancreatic secretion by intravenous infusion of caerulein resulted in a progressive reduction of the amylase concentration and still more of the dry residue content of pancreatic juice. The bicarbonate concentration in pancreatic juice produced by caerulein was similar to that observed in juice secreted following pancreozymin administration or following other stimuli causing the same rate of flow of pancreatic juice.2. On a molar basis, caerulein was 25-30 times as active as human gastrin I and 3-6 times as active as cholecystokinin-pancreozymin. The presence in the molecule of caerulein of a sulphated tyrosyl residue at position 4 of the decapeptide (position 7 starting from the C-terminus) was a necessary prerequisite for the manifestation of the cholecystokinin-pancreozymin-like actions of caerulein. The C-terminal heptapeptide of caerulein retained much of the activity of the intact caerulein molecule.3. At high dose levels (50-200 ng/kg in the dog, 1 mug/kg in the rat, by rapid intravenous injection) caerulein stimulated the flow of hepatic bile in the dog and the rat. The dry residue of the bile and the cholesterol concentration were appreciably greater in rats treated with caerulein than in control rats.4. The activity spectrum of caerulein was identical with that of cholecystokinin-pancreozymin. This is readily explained on the basis of the almost identical structure of the C-terminal octapeptide of the two peptides.5. Caerulein and some caerulein-like peptides may be considered as model peptides, capable of being substituted for cholecystokinin-pancreozymin in all the possible experimental and clinical uses of the duodenal hormone, with the important advantage that they are more easily available.6. The question is raised whether cholecystokinin-pancreozymin obtained from the duodenum by acid extraction is the authentic hormone or rather a carrier polypeptide from which a smaller active peptide may be set free, when needed, into the circulation.