Gut peptide YY (PYY) plays an important role in regulating metabolism and is expressed during the ontogeny of the pancreas. However, its biological role during endocrine cell formation is not fully understood, and its role, if any, during pancreatic regeneration in the adult has not yet been explored. The knowledge of factors involved in beta cell renewal in adult animals is clearly relevant for the design of treatment strategies for type 1 diabetes. We therefore sought to determine if observations during fetal pancreas formation also apply to pancreatic growth in adult animals. Indeed, we have found marked expansion of the PYY-expressing population during pancreatic regeneration. In addition, we demonstrate the presence of cells co-expressing PYY and the critical pancreatic transcription factor pancreatic duodenal homeobox1 (PDX-1). Interestingly, these cells also co-expressed specific islet hormones during pancreatic development and regrowth, suggesting a developmental relationship. Furthermore, we have found that PYY can act in concert with IGF-1 to stimulate cellular responsiveness in pancreatic epithelial cells in vitro. Our data suggest that PYY may be a mediator of islet cell development, as well as a cofactor for growth factor responses, not only during fetal pancreas formation but also during regeneration in adult animals.