1 The antihypertensive agent moxonidine, an imidazoline I i -receptor agonist, also induces hypophagia and lowers body weight in the obese spontaneously hypertensive rat, but the central mediation of this action and the neuronal pathways that moxonidine may interact with are not known. We studied whether moxonidine has anti-obesity e ects in the genetically-obese and insulinresistant fa/fa Zucker rat, and whether these are mediated through inhibition of the hypothalamic neuropeptide Y (NPY) neurones. 2 Lean and obese Zucker rats were given moxonidine (3 mg kg 71 day 71 ) or saline by gavage for 21 days. 3 Moxonidine decreased food intake throughout by 20% in obese rats (P50.001) and by 8% in lean rats (P50.001), and reduced weight gain that ®nal body weight was 15% lower in obese (P50.001) and 7% lower in lean (P50.01) rats than their untreated controls. Plasma insulin and leptin levels were decreased in moxonidine-treated obese rats (P50.01 and P50.05), but unchanged in treated lean rats. Uncoupling protein-1 gene expression in brown adipose tissue was stimulated by 40 ± 50% (P40.05) in both obese and lean animals given moxonidine. Obese animals given moxonidine showed a 37% reduction in hypothalamic NPY mRNA levels (P=0.01), together with signi®cantly increased NPY concentrations in the paraventricular nucleus (P50.05), but no changes in the arcuate nucleus or other nuclei; this is consistent with reduced NPY synthesis in the arcuate nucleus and blocked release of NPY in the paraventricular nucleus. In lean animals, moxonidine did not a ect NPY levels or NPY mRNA. 4 The hypophagic, thermogenic and anti-obesity e ects of moxonidine in obese Zucker rats may be partly due to inhibition of the NPY neurones, whose inappropriate overactivity may underlie obesity in this model.