Agouti-related protein (AGRP) is an orexigenic neuropeptide that acts via central melanocortin receptors, and whose messenger RNA (mRNA) levels are elevated in leptin-deficient mice. Fasting associated with a decline in circulating leptin normally causes a 15-fold elevation of hypothalamic Agrp mRNA levels but has no effect in leptin-deficient mice. Chronic hyperleptinemia associated with the tubby and Cpe^fat mutations has no effect on Agrp mRNA levels, but short term leptin administration causes a 17% reduction of Agrp mRNA levels in nonmutant mice and a 700% reduction in leptin-deficient mice. In young nonobese animals, melanocortin receptor blockade associated with the A^y mutation causes complete resistance to leptin-induced weight loss. Dual in situ hybridization reveals that Agrp-expressing neurons in the medial portion of the arcuate nucleus constitute a subpopulation different from Pomc-expressing neurons, and that a significant proportion of Agrp-expressing neurons (10–25%) coexpresses the leptin receptor, Lepr-b. Immunocytochemistry confirms distinct locations of AGRP- and POMC-expressing cell bodies, but reveals an overlapping distribution of their terminal fields in the arcuate nucleus, the paraventricular hypothalamus, and the dorsomedial hypothalamus. These results suggest that in the fed state, AGRP is normally suppressed by leptin, and that release of this suppression during fasting leads to increased ingestive behavior.