Leptin plays a pivotal role in the regulation of energy homeostasis and neuroendocrine functions, and increasing evidence indicates that leptin acts on the brain to mediate many of these effects. Recent data have also suggested that leptin influences brain development during early postnatal life. Here we examined the distribution of cells that express mRNA encoding the long form of the leptin receptor (LepRb) in postnatal and adult mouse brains by using in situ hybridization. In both adults and neonates, LepRb mRNA was largely restricted to regions known to control energy balance. Labeled cells were found in the arcuate, ventromedial, and dorsomedial nuclei of the hypothalamus as well as in the lateral hypothalamic area. Heavily labeled cells were also found in the median preoptic and ventral premammillary nuclei, two hypothalamic nuclei that are known to control reproduction. Moreover, during postnatal and adult life, clearly labeled cells were found in extrahypothalamic autonomic control sites such as the nucleus of the tractus solitarius. Importantly, this receptor can induce intracellular signaling because peripheral injection of leptin caused STAT3 phosphorylation in most sites in which LepRb mRNA was expressed. LepRb mRNA was also transiently elevated in certain regions of the postnatal mouse brain, such as the cortex, hippocampus, and laterodorsal nucleus of the thalamus. Taken together, these observations are consistent with the proposed roles of leptin in feeding and neuroendocrine regulation. They also identify regions where LepRb mRNA is expressed during early postnatal life and suggest new roles for leptin in the nervous system during development. J. Comp. Neurol. 518:459–476, 2010. © 2009 Wiley‐Liss, Inc.