Interleukin (IL)-6 is a pro-inflammatory cytokine that also affects metabolic function because IL-6 depleted (IL-6−/−) mice develop late-onset obesity. IL-6 appears to act in the central nervous system, presumably in the hypothalamus, to increase energy expenditure that appears to involve stimulation of the sympathetic nervous system. In the present study, we explored possible central mechanisms for the effects exerted by IL-6 on body fat. Therefore, we measured the effects of IL-6 depletion in IL-6−/− mice on expression of key hypothalamic peptide genes involved in energy balance by the real time polymerase chain reaction. Additionally, co-localisation between such peptides and IL-6 receptor α was investigated by immunohistochemistry. IL-6 deficiency decreased the expression of several peptides found in the paraventricular nucleus (PVN), which is a nucleus that has been attributed an adipostatic function. For example, corticotrophin-releasing hormone (CRH), which is reported to stimulate the sympathetic nervous system, was decreased by 40% in older IL-6−/− mice. Oxytocin, which is reported to prevent obesity, was also decreased in older IL-6−/− animals, as was arginine vasopressin (AVP). The IL-6 receptor α was abundantly expressed in the PVN, but also in the supraoptic nucleus, and was shown to be co-expressed to a high extent with CRH, AVP, oxytocin and thyrotrophin-releasing hormone. These data indicate that depletion of endogenous IL-6, a body fat suppressing cytokine, is associated with the decreased expression of CRH and oxytocin (i.e. energy balance regulating peptides) as well as AVP in the PVN. Because IL-6 receptor α is co-expressed with CRH, oxytocin and AVP, IL-6 could stimulate the expression of these peptides directly.