Cytokine-mediated anorexia is a component of "sickness behavior" and presents a significant obstacle in the treatment of chronic illnesses. We hypothesized an involvement of the hypothalamic arcuate nucleus (ARH) in mediating the anorexic effects of a systemic interleukin-1 (IL-1) challenge based on its content of peptidergic neurons involved in feeding, its expression of IL-1 receptors and its sensitivity to systemic IL-1. IL-1 (6 microg/kg, i.v.) was found to induce Fos expression in both pro-opiomelanocortin- and neuropeptide Y-expressing neurons in and around the ARH. Contrary to expectations, rats that had sustained lesions of the arcuate nucleus, produced by neonatal monosodium glutamate treatment, displayed a more pronounced suppression (by 25%) of food intake than nonlesioned controls when treated with IL-1 after a 20 hr fast. To confirm and further characterize this unexpected result, a second ablation method was used in a similar paradigm. Animals bearing knife cuts designed to sever major ARH projections displayed an even more accentuated loss of appetite (by 60%, relative to controls) in response to systemic IL-1. This effect exhibited at least some degree of specificity, because the knife cuts did not alter either IL-1 effects on another centrally mediated acute phase response (fever) or the anorexia produced by an alternate agent, fenfluramine. These results fail to support the hypothesized ARH mediation of IL-1-induced anorexia and may suggest rather that the net output of this cell group may serve normally to restrain cytokine-induced reductions in food intake.