The melanotropic peptides were discovered and named because of their vital importance in the control of pigmentation of frogs, tadpoles, chameleons, lizards, and fish. Melanotropic peptides can also darken the skin of human beings. The occurrence of these potent agents at extrapituitary sites such as the mammalian central nervous system, raises the question as to whether they affect structures other than pigment cells. The amino acid sequences of the melanotropic peptides occur in the precursor molecule, pro-opiomelanocortin. Intracellular processing of pro-opiomelanocortin in the pituitary gland can lead to the release of α-, β-, and γ-melanocyte-stimulating hormone. The experimental modifications in peptide structure by Victor J. Hruby and associates have demonstrated that hormonal activity can be increased dramatically. There are numerous ways of changing peptide molecules. The work with a-MSH has led the field.