The hormone leptin acts on the brain to regulate feeding, metabolism, and reproduction; however, its cellular targets and molecular mechanisms of action remain to be fully elucidated. The melanocortins, which are derived from the precursor proopiomelanocortin (POMC), are also implicated in the physiological regulation of body weight. POMC-containing neurons express the leptin receptor, and thus it is conceivable that the POMC gene itself may be part of the signaling pathway involved in leptin's action on the brain. Using in situ hybridization and computerized image analysis, we tested the hypothesis that the POMC gene is a target for regulation by leptin by comparing cellular levels of POMC mRNA in the hypothalamus among groups of leptin-deficient (ob/ob) mice, leptin-treated ob/ob mice, and wild-type controls. POMC mRNA levels were significantly reduced throughout the arcuate nucleus in vehicle-treated ob/ob mice relative to wild-type controls, whereas POMC mRNA levels in leptin-treated ob/ob mice were indistinguishable from wild-type controls. These observations suggest that one or more products of POMC serve as an integrative link between leptin and the central mechanisms governing body weight regulation and reproduction.