Neuropeptide Y, peptide YY, and pancreatic polypeptide are structurally related peptides that are considered to play a role in the regulation of pancreatic secretion and blood flow. Several receptor subtypes for these peptides have been identified, and the Y1, Y2, Y4/PP1, Y5, and Y5/PP2/Y2b receptors are cloned. We have prepared polyclonal peptide antibodies that recognize the Y1 receptor and now report on its localization in the adult and developing rat pancreas. In the adult pancreas, Y1 receptors were detected both in some centroacinar and intralobular duct cells and in endothelial cells. In the developing pancreas (E12.5-E16.5), Y1 receptor immunoreactivity was observed in numerous nonendocrine epithelial cells. These cells occurred in the immediate vicinity of peptide YY-positive endocrine cells. At E16.5, a fraction of these Y1 receptor-containing cells co-stored amylase. One day later, Y1 receptor immunoreactivity became restricted to pancreatic duct-like cells that occurred in close proximity to peptide YY cells. In fetal rats, intense Y1 receptor staining was also observed in endothelial cells. These observations, together with the finding of early pancreatic peptide YY expression, suggest that peptide YY produced by fetal endocrine cells may exert an action on exocrine cells, duct cells and endothelial cells during development.