The central melanocortin system has emerged as a potential regulator of food intake. This action of melanocortins appears to occur through intrahypothalamic, melanocortin-containing projections, including those from the arcuate to the paraventricular nucleus (PVN). Although the complexity of feeding behavior and the long duration of the effects of melanocortins on food intake suggest changes in gene expression, the mechanism by which such changes occur has been elusive. In the present report, we describe experiments using in vitro and in vivo approaches to demonstrate melanocortin-induced phosphorylation (activation) of members of the mitogen-activated protein kinase (MAPK) family of transcription factors. First, application of the melanocortin agonist MTII to COS-1 cells resulted in an increase in phosphorylated MAPK after the cells were transfected with the melanocortin type 4 receptor (MC4-R), but not the type 3 receptor. Formation of cAMP, however, was observed when either receptor subtype was transfected. Subsequent experiments revealed that the effect of MTII on MAPK activation in MC4-R-transfected cells was dose-dependent and was maximal after 10 min of MTII exposure. Second, central injections of MTII increased the number of phospho-MAPK-immunoreactive cells in the rat PVN compared to vehicle-injected animals. When coupled with immunohistochemical identification of PVN neurons containing oxytocin, a clear segregation was apparent, allowing for a precise anatomical description of the pattern of activated MAPK within the PVN. These data are the first to suggest a differential coupling of MC4-R and may describe a mechanism through which the long-term and persistent behavioral actions of melanocortins are mediated.