The aim of this research is to determine the function of the hormone, pancreatic polypeptide (PP). Using (125I) labelled PP and competitive binding methods applied in vivo, pancreas, intestine, spleen and bone marrow have been identified as target organs. Only PP labelled at the amino terminal position with the Bolton-Hunter reagent will compete with unlabelled PP in these experiments. Further studies with these target organs are in progress. The effect of PP upon basal and hormone stimulated amylase secretion by isolated pancreatic acini is being investigated. Radioautographs at the light microscopy and electron microscope levels are being used to identify the cell type in intestine, spleen and bone marrow wherever (125I) avian PP binds. Antiserum to avian PP but not to bovine, canine, or human PP when applied in indirect immunofluorescence assays detects a substance in neurons and fibers of rat (and other mammals) brain and spinal cord and especially sympathetic ganglia. This is apparently a new peptide neurotransmitter with amino acid sequence similar to that of avian PP. Accordingly, the antigenic site of a PP is being determined an an attempt is being made to isolate the peptide from brain. Isolation and characterization of PP from rat and alligator pancreas and catfish islets is in progress.