1. 1. The darkening actions of MCH (melanin concentrating hormone), α-MSH and the synthetic analog [Nle 4, d-Phe 7-α-MSH on the toad, Bufo ictericus ictericus, melanophores were studied regarding the role of calcium in the hormone receptor coupling, signal transduction and intracellular pigment translocation. 2. 2. In the absence of external calcium, MCH and both melanotropins still elicit maximal skin darkening. 3. 3. Verapamil, a calcium-channel blocker, completely abolishes the α-MSH-induced response and partially inhibits MCH-induced darkening, although the calcium carrier, ionophore A 23187, was unable to promote any pigment translocation. 4. 4. Since darkening responses promoted by cyclic nucleotides proceeded normally in the presence of verapamil and extracellular calcium was not necessary for melanotropin dispersing action, it is suggested that the blocking activity obtained with verapamil is probably due to an impairment of the Ca 2+-dependent adenylate cyclase activity. 5. 5. Reversal of melanotropin-induced darkening could be obtained with melatonin, in both normal and Ca 2+-free Ringer, whereas MCH darkening is reversed by melatonin only in the absence of calcium. 6. 6. The results seem to indicate that calcium is not required for hormone receptor binding and pigment migration, whereas it is specifically needed for signal transduction.