GH feeds back on the hypothalamus and regulates its own secretion. We have previously shown that systemic administration of GH induces expression of the c-fos gene, a marker of neuronal activity, on the hypothalamic neuropeptide Y(NPY) and somatostatin neurons in rats. We argued that if GH were to act directly on NPY neurons, NPY neurons should express the GH receptor (GHR) gene. To test this hypothesis, coronal sections of the medial basal hypothalamus from adult male Wistar rats were processed by double label in situ hybridization using a 35S-labeled NPY complementary RNA probe and a digoxigenin-labeled GHR complementary RNA probe. In the medial basal hypothalamus, NPY messenger RNA (mRNA) was observed in the arcuate nucleus (ARC) and the dorsomedial nucleus. The majority (95%) of NPY mRNA-containing cells in the ARC expressed the GHR gene, whereas no NPY mRNA-containing cells in the dorsomedial nucleus expressed the GHR gene. These findings suggest that NPY neurons in the ARC mediate the feedback effect of GH on the hypothalamus.