In adults, the adipocyte-derived hormone, leptin, regulates food intake and body weight principally via the hypothalamic arcuate nucleus (ARC). During early postnatal development, leptin functions to promote the outgrowth of neuronal projections from the ARC, while a selective insensitivity to the effects of leptin on food intake appears to exist. To investigate the mechanisms underlying the inability of leptin to regulate food intake during early development, leptin signalling was analysed both in vitro using primary cultures of rat embryonic ARC neurones and in vivo by challenging early postnatal rats with leptin. In neuronal cultures, despite the presence of key components of the leptin signalling pathway, no detectable activation of either signal transducer and activator of transcription 3 (STAT3) or the mitogen-activated kinase (MAPK) pathways by leptin was detected. However, leptin down-regulated mRNA levels of pro-opiomelanocortin (POMC) and neuropeptide Y (NPY) and decreased somatostatin secretion. Leptin challenge in vivo at postnatal days 7 (P7), P14, P21 and P28 revealed that, in contrast to adult and P28 rats, mRNA levels of NPY, POMC, agouti-related peptide (AgRP) and cocaine- and amphetamine-regulated transcript (CART) were largely unaffected at P7, P14 and P21. Furthermore, leptin stimulation increased the suppressor of cytokine signalling-3 (SOCS-3) mRNA levels at P14, P21 and P28 in several hypothalamic nuclei, but not at P7, indicating that selective leptin insensitivity in the hypothalamus is coupled to developmental shifts in leptin receptor signalling. Thus, the present study defines the onset of leptin sensitivity in the regulation of energy homeostasis in the developing hypothalamus.