Mouse lines with targeted disruption of the cocaine amphetamine-related transcript (CART), melanocortin receptor 3 (MCR3), or melanocortin receptor 4 (MCR4) were used to assess the role of each component in mediating the anorectic and metabolic effects of leptin, and in regulating the partitioning of nutrient energy between fat and protein deposition. Leptin was administered over a 3 day period using either intraperitoneal or intracerebroventricular routes of injection. The absence of MCR4 blocked leptin's ability to increase UCP1 mRNA in both brown and white adipose tissue, but not its ability to reduce food consumption. In contrast, deletion of MCR3 compromised leptin's ability to reduce food consumption, but not its ability to reduce fat deposition or increase UCP1 expression in adipose tissue. Leptin-dependent effects on food consumption and adipocyte gene expression were unaffected by the absence of CART. Repeated measures of body composition over time indicate that the absence of either MCR3 or MCR4, but not CART, increased lipid deposition and produced comparable degrees of adiposity in both lines. Moreover, modest increases in fat content of the diet (4 to 11%) accentuated fat deposition and produced a rapid and comparable 10-12% increase in % body fat in both genotypes. The results indicate that nutrient partitioning, as well as the anorectic and metabolic responses to leptin, are dependent on integrated but separable inputs from the melanocortin 3 and 4 receptor subtypes.