Orexin-A is a unique hypothalamic neuropeptide that stimulates both food intake and energy expenditure, although orexigenic peptides usually have coordinated effects on fat storage by increasing food intake and decreasing energy expenditure. Here we investigated the site of action of orexin-A-induced thermogenesis in urethane-anesthetized rats. Microinjection of 1-10 pmol orexin-A into the arcuate nucleus (Arc) specifically increased whole-body O(2) consumption (VO(2)), an index of energy expenditure; whereas it had no effect on VO(2) when injected into the paraventricular nucleus (PVN), dorsomedial nucleus (DMH), lateral hypothalamus (LH), ventromedial nucleus (VMH) or medial preoptic nucleus (MPO) of the hypothalamus or into in the paraventricular thalamic nucleus (PVT) or pontine locus coeruleus (LC). VO(2) increased immediately after an orexin-A injection into the Arc, and this increase was accompanied by a simultaneous tachycardiac response and a gradual increase in colonic temperature (T(co)), whereas an injection of the saline vehicle into the Arc had no effect. The effective dose of orexin-A into the Arc was 10 times less than that into the cerebral ventricle to induce a similar level of response. In addition, intracerebroventricular administration of orexin-A (100 pmol) elicited a significantly smaller VO(2) response in Arc-lesioned rats than that in sham-operated control rats. These results suggest that the orexin-induced energy expenditure is mediated, at least in part, by the Arc.