Agouti-related protein (Agrp) is present in rat and human hypothalamus and is structurally related to agouti protein. Over-expression of either of these proteins results in obesity. However the effect of exogenous Agrp and its in vivo interaction with alpha-melanocyte stimulating hormone (αMSH), the likely endogenous melanocortin 3 and 4 receptor (MC3-R and MC4-R) agonist, have not been demonstrated. We report that 1 nmol of Agrp(83-132), a C-terminal fragment of Agrp, when administered intracerebroventricularly (ICV) into rats, increased food intake over a 24-h period (23.0±1.4 g saline vs 32.9±2.3 g Agrp, p<0.05). The hyperphagia was similar to that seen when 1 nmol of the synthetic MC3-R and MC4-R antagonist SHU9119 was given ICV (19.6±1.8 g saline vs 32.5±1.7 g SHU9119, p<0.001). Both Agrp(83-132) and SHU9119 blocked the reduction in 1-h food intake of ICV αMSH at the beginning of the dark phase. This effect occurred independently of whether the antagonists were administered simultaneously, or nine hours prior, to the αMSH. We have also shown Agrp(83-132) is an antagonist at the MC3-R and MC4-R, with similar inhibition of cAMP activation to that previously reported for the full length peptide. In conclusion, Agrp(83-132) administered ICV increases feeding with long lasting effects and is able to inhibit the action ofα MSH. This interaction may be mediated by the MC3-R and/or MC4-R.