Neuropeptide Y (NPY) is a potent orexigenic peptide. In the normal adult rat, hypothalamic NPY mRNA expression is limited to the arcuate nucleus (ARH). The purpose of this study was to characterize the developmental expression of NPY mRNA in the hypothalamus of the rat. In contrast to the normal adult rat, NPY mRNA was observed in the ARH, the dorsomedial hypothalamic nucleus (DMH) and the perifornical region (PFR) during development. NPY mRNA expression in all three regions increased progressively from postnatal days 0-4 (P0-4) to reach maximum levels at P16 and subsequently decreased to near adult levels by P30. The unique expression of NPY mRNA in the PFR and DMH may be important in establishing the proper management of energy homeostasis and body weight in the adult animal.