Pancreastatin is a recently isolated pancreatic peptide which has been shown to be a potent inhibitor of insulin release in a perfused rat pancreas assay (1). In order to study the regulation of pancreastatin mRNA and to examine the primary structure of its precursor, genomic and cDNA clones corresponding to pancreastatin have been isolated from rat genomic and pancreatic libraries. The number of genes isolated, along with southern blot analyses, suggested that pancreastatin may be part of a gene family. We examined the regulation of pancreastatin expression by insulin and streptozotocin treatment. Pancreastin mRNA levels in rats are altered in response to insulin and streptozotocin.