Leptin is involved in the hypothalamic control of food intake and body weight. Fos immunohistochemistry has been used to functionally map leptin target neurons involved in these regulatory processes. However, only a subset of hypothalamic neurons expressing the long form of the leptin receptor (Ob-Rb) also coexpress the neuronal activation marker Fos after leptin stimulation. To functionally map all leptin target neurons, regardless of whether leptin-mediated neuronal activation or inhibition occurs, we immunohistochemically investigated the leptin-induced nuclear translocation of the signal transducer and activator of transcription molecule STAT3, which represents a crucial step in the regulation of leptin-dependent gene expression. As proven by colocalization studies with the nuclear 4',6-diamidino-2-phenylindole dilactate stain, intracerebroventricular leptin treatment, but not intracerebroventricular application of pyrogen-free saline, induced a time-dependent nuclear translocation of STAT3 immunoreactivity in hypothalamic nuclei, with strong nuclear STAT3 signals detectable in the arcuate nucleus, the lateral hypothalamus, and the ventromedial and dorsomedial hypothalamic nuclei. This leptin-induced STAT3 translocation pattern proved to be distinct from that induced by interleukin-6, another cytokine using STAT3 in its signaling pathway. Combined immunohistochemical STAT3 and Fos detection after leptin treatment revealed a higher number of STAT3-positive than Fos-positive cell nuclei in the aforementioned hypothalamic structures and showed that Fos immunoreactivity colocalized only in a subset of all leptin-responsive STAT3 nuclei. These results suggest that the detection of nuclear STAT3 immunoreactivity represents a new neuroanatomical tool to functionally map central leptin actions. They further support the importance of ventrally located caudal hypothalamic structures representing the main leptin targets involved in body weight regulation.