A single injection of the melanotropin analog [4-norleucine, 7-D-phenylalanine]-alpha-melanotropin into frogs (Rana pipiens) caused near maximum darkening of the skins of the frogs for at least 6 weeks. Injections of the natural hormone alpha-melanotropin or of the analog [Nle4]-alpha-melanotropin also caused darkening, but this effect lasted only a few days. Morphological examination of the skins of frogs injected with [Nle4, D-Phe7]-alpha-melanotropin revealed that both dermal and epidermal melanophores were dispersed during the entire 6-week period. In vitro [Nle4, D-Phe7]-alpha-melanotropin also causes prolonged darkening of the skin of the lizard Anolis carolinensis. In the absence of the melanotropin, skins previously darkened with the analog could be lightened by removal of calcium from the incubation medium but could then be redarkened by adding calcium. The cycle could be repeated indefinitely without addition of melanotropin. These results demonstrate the role of calcium in receptor signal transduction and the prolonged biological effects of [Nle4, D-Phe7]-alpha-melanotropin long after its removal from the assay medium.