This chapter explains the mechanisms by which melanotropic peptides regulate melanosome movements within melanocytes. α-Melanotropin (MSH) synthesized and released by the vertebrate pituitary gland, acts on pigment cells within the skin. Melanotropic peptides play essential roles in the control of melanocytes and melanin pigmentation of the skin in many animals. Melanogenesis and melanosome formation continue as important functions of epidermal melanocytes. Mature melanosomes are continuously released into adjacent keratinocytes of the epidermis and are replaced within the melanocytes by newly synthesized melanosomes. MSH also darkens skins in vitro and, as in vivo, these darkening results from melanosome dispersion within melanocytes. Frog skins darken in response to theophylline and other methylxanthines which are known inhibitors of phosphodiesterase, an enzyme which degrades cyclic adenosine monophosphate (cAMP) within cells. The intracellular actions of cAMP are probably mediated through activation of a cAMP-dependent protein kinase. The receptors, through which MSH and melanin-concentrating hormone mediate their actions, are separate.