The melanocortin-4 receptor (MC4R) is a G-protein coupled receptor (GPCR) that is expressed in the central nervous system and has a role in regulating energy homeostasis and obesity. Up to a remarkable 6% of morbidly obese adults and children studied possess single nucleotide polymorphisms (SNPs) of the MC4R. Upon stimulation by agonist, the MC4R signals through the intracellular adenylate cyclase signal transduction pathway. Posttranslational modification of the pro-opiomelanocortin (POMC) gene transcript results in the generation of several endogenous melanocortin receptor agonists including alpha-, beta-, gamma-melanocyte stimulating hormones (MSH) and adrenocorticotropin (ACTH) ligands. The endogenous MC4R antagonist, agouti-related protein (AGRP), is expressed in the brain and is only one of two naturally occurring antagonists of GPCRs identified to date. Herein, we have generated 40 hMC4 polymorphic receptors and evaluated their cell surface expression by flow cytometry as well as pharmacologically characterized their functionality using the endogenous agonists alpha-MSH, beta-MSH, gamma2-MSH, ACTH(1-24), the antagonist hAGRP(87-132), and the synthetic agonists NDP-MSH and MTII. This is the first study in which polymorphic hMC4Rs have been pharmacologically characterized simultaneously with multiple endogenous ligands. Interestingly, at the N97D, L106P, and C271Y hMC4Rs beta-MSH was more potent than the other endogenous agonists alpha-MSH, gamma2-MSH, ACTH(1-24). The S58C and R165Q/W hMC4Rs possessed significantly reduced endogenous agonist potency (15- to 90-fold), but the synthetic ligands NDP-MSH and MTII possessed only 2-9-fold reduced potency as compared to the wild-type receptor, suggesting their potential as therapeutic ligands to treat individuals with these polymorphisms.