Diabetes and obesity are approaching epidemic prevalence in the United States. Neuropathic pain is a condition that makes everyday life unbearable for those afflicted, and currently lacks effective treatment. The human melanocortin system is involved in all three conditions as well as sexual impotency and skin pigmentation. Our lab is on the forefront of melanocortin research and has worked the problem for over 40 years, developing the nonselective melanocortin agonist, MT-II. The current research is a systematic examination of the structure activity relationships between side-chain methylated MT-II analogues and the melanocortin receptor subtypes 1, 3, 4 and 5. Binding affinity was tested via radioligand binding assays with the MT-II analogues in competition with [125I]NDP-alpha-MSH for receptors expressed on the surface of stably transfected HEK293 cell lines. Biological activity was assessed by measuring the production of cAMP by transfected cells after stimulation with analogues. We found that methylated Histidine, D-Phenylalanine and alpha-N-methylated Lysine side chains resulted in an hMC3R selective full antagonist with an IC50 of 20nM, while methylation of the D-Phe side chain alone resulted in an hMC3R selective partial agonist with an IC50 of 75nM and EC50 of 120nM. PA2 values will demonstrate competition in the full antagonist. A confocal imaging study of the internalization of fluorescently tagged MT-II analogues will confirm agonist and antagonist activity. NMR structures of the analogues are being solved.