The immune-modulating cytokine interleukin-6 (IL-6) is expressed both in adipose tissue and centrally in hypothalamic nuclei that regulate body composition. We investigated the impact of loss of IL-6 on body composition in mice lacking the gene encoding IL-6 (Il6-/- mice) and found that they developed mature-onset obesity that was partly reversed by IL-6 replacement. The obese Il6-/- mice had disturbed carbohydrate and lipid metabolism, increased leptin levels and decreased responsiveness to leptin treatment. To investigate the possible mechanism and site of action of the anti-obesity effect of IL-6, we injected rats centrally and peripherally with IL-6 at low doses. Intracerebroventricular, but not intraperitoneal IL-6 treatment increased energy expenditure. In conclusion, centrally acting IL-6 exerts anti-obesity effects in rodents.