A 13-amino acid peptide derived from proteolytic cleavage of adrenocorticotropic hormone, the N-terminal segment of ACTH. ACTH (1-13) is amidated at the C-terminal to form ACTH (1-13)NH2 which in turn is acetylated to form alpha-msh in the secretory granules. alpha-msh stimulates the synthesis and distribution of MELANIN in melanocytes in mammals and melanophores in lower vertebrates.