Neuropeptide Y (NPY), a neuropeptide abundantly expressed in the brain, has been implicated in the regulation of the hypothalamo-pituitary-somatotropic-axis and the hypothalamo-pituitary-gonadotropic-axis. Elevated hypothalamic NPY expression, such as that occurs during fasting, is known to inhibit both of these axes. However, it is not known which Y receptor(s) mediate these effects. Here we demonstrate, using Y receptor knockout mice, that Y2 and Y4 receptors are separately involved in the regulation of these axes. Fasting-induced inhibition of hypothalamic growth hormone releasing hormone (GHRH) mRNA expression and reduction of circulating insulin-like growth factors levels were observed in wildtype and Y4^-/- mice but not in Y2^-/- or Y2^-/-Y4^-/- mice. In contrast, fasting-induced reduction of gonadotropin-releasing-hormone (GnRH) expression in the medial preoptic area and testis testosterone content were abolished in the absence of Y4 receptors. Co-localization of Y2 receptors and GHRH in the arcuate nucleus (Arc) suggests that GHRH mRNA expression in this region might be directly regulated by Y2 receptors. Indeed, hypothalamic-specific deletion of Y2 receptors in conditional knockout mice prevented the fasting-induced reduction in Arc GHRH mRNA expression. On the other hand, fasting-induced decrease in GnRH mRNA expression in the MPA is more likely indirectly influenced by Y4 receptors as no Y4 receptors could be detected on GnRH neurons in this region. Together these data show that fasting inhibits the somatotropic axis via direct action on Y2 receptors in the Arc, and indirectly inhibits the gonadotropic axis via Y4 receptors.